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PubMed2001

Diversity, origin, and distribution of retrotransposons (gypsy and copia) in conifers.

We examined the diversity, evolution, and genomic organization of retroelements in a wide range of gymnosperms. In total, 165 fragments of the reverse transcriptase (RT) gene domain were sequenced from PCR products using newly designed primers for gypsy-like retrotransposons and well-known primers for copia-like retrotransposons; representatives of long interspersed nuclear element (LINE) retroposons were also found. Gypsy and copia-like retroelements are a major component of the gymnosperm genome, and in situ hybridization showed that individual element families were widespread across the chromosomes, consistent with dispersion and amplification via an RNA intermediate. Most of the retroelement families were widely distributed among the gymnosperms, including species with wide taxonomic separation from the Northern and Southern Hemispheres. When the gymnosperm sequences were analyzed together with retroelements from other species, the monophyletic origin of plant copia, gypsy, and LINE groups was well supported, with an additional clade including badnaviral and other, probably virus-related, plant sequences as well as animal and fungal gypsy elements. Plant retroelements showed high diversity within the phylogenetic trees of both copia and gypsy RT domains, with, for example, retroelement sequences from Arabidopsis thaliana being present in many supported groupings. No primary branches divided major taxonomic clades such as angiosperms, monocotyledons, gymnosperms, or conifers or (based on smaller samples) ferns, Gnetales, or Sphenopsida (Equisetum), suggesting that much of the existing diversity was present early in plant evolution, or perhaps that horizontal transfer of sequences has occurred. Within the phylogenetic trees for both gypsy and copia, two clearly monophyletic gymnosperm/conifer clades were revealed, providing evidence against recent horizontal transfer. The results put the evolution of the large and relatively conserved genome structure of gymnosperms into the context of the diversity of other groups of plants.

Animals↗

PubMed2005

Ribonuclease H evolution in retrotransposable elements.

Eukaryotic and prokaryotic genomes encode either Type I or Type II Ribonuclease H (RNH) which is important for processing RNA primers that prime DNA replication in almost all organisms. This review highlights the important role that Type I RNH plays in the life cycle of many retroelements, and its utility in tracing early events in retroelement evolution. Many retroelements utilize host genome-encoded RNH, but several lineages of retroelements, including some non-LTR retroposons and all LTR retrotransposons, encode their own RNH domains. Examination of these RNH domains suggests that all LTR retrotransposons acquired an enzymatically weak RNH domain that is missing an important catalytic residue found in all other RNH enzymes. We propose that this reduced activity is essential to ensure correct processing of the polypurine tract (PPT), which is an important step in the life cycle of these retrotransposons. Vertebrate retroviruses appear to have reacquired their RNH domains, which are catalytically more active, but their ancestral RNH domains (found in other LTR retrotransposons) have degenerated to give rise to the tether domains unique to vertebrate retroviruses. The tether domain may serve to control the more active RNH domain of vertebrate retroviruses. Phylogenetic analysis of the RNH domains is also useful to "date" the relative ages of LTR and non-LTR retroelements. It appears that all LTR retrotransposons are as old as, or younger than, the "youngest" lineages of non-LTR retroelements, suggesting that LTR retrotransposons arose late in eukaryotes.

Animals↗

PubMed2005

Diversity and clustered distribution of retrotransposable elements in the compact genome of the pufferfish Tetraodon nigroviridis.

We report the characterization and chromosomal distribution of retroelements in the compact genome of the pufferfish Tetraodon nigroviridis. We have reconstructed partial/complete retroelement sequences, established their phylogenetic relationship to other known eukaryotic retrotransposons, and performed double-color FISH analyses to gain new insights into their patterns of chromosomal distribution. We could identify 43 different reverse transcriptase retrotransposons belonging to the three major known subclasses (14 non-LTR retrotransposons from seven clades, 25 LTR retrotransposons representing the five major known groups, and four Penelope-like elements), and well as two SINEs (non-autonomous retroelements). Such a diversity of retrotransposable elements, which seems to be relatively common in fish but not in mammals, is astonishing in such a compact genome. The total number of retroelements was approximately 3000, roughly representing only 2.6% of the genome of T. nigroviridis. This is much less than in other vertebrate genomes, reflecting the compact nature of the genome of this pufferfish. Major differences in copy number were observed between different clades, indicating differential success in invading and persisting in the genome. Some retroelements displayed evidence of recent activity. Finally, FISH analysis showed that retrotransposable elements preferentially accumulate in specific heterochromatic regions of the genome of T. nigroviridis, revealing a degree of genomic compartmentalization not observed in the human genome.

Animals↗

PubMed2004

Genomic neighborhoods for Arabidopsis retrotransposons: a role for targeted integration in the distribution of the Metaviridae.

BACKGROUND: Retrotransposons are an abundant component of eukaryotic genomes. The high quality of the Arabidopsis thaliana genome sequence makes it possible to comprehensively characterize retroelement populations and explore factors that contribute to their genomic distribution. RESULTS: We identified the full complement of A. thaliana long terminal repeat (LTR) retroelements using RetroMap, a software tool that iteratively searches genome sequences for reverse transcriptases and then defines retroelement insertions. Relative ages of full-length elements were estimated by assessing sequence divergence between LTRs: the Pseudoviridae were significantly younger than the Metaviridae. All retroelement insertions were mapped onto the genome sequence and their distribution was distinctly non-uniform. Although both Pseudoviridae and Metaviridae tend to cluster within pericentromeric heterochromatin, this association is significantly more pronounced for all three Metaviridae sublineages (Metavirus, Tat and Athila). Among these, Tat and Athila are strictly associated with pericentromeric heterochromatin. CONCLUSIONS: The non-uniform genomic distribution of the Pseudoviridae and the Metaviridae can be explained by a variety of factors including target-site bias, selection against integration into euchromatin and pericentromeric accumulation of elements as a result of suppression of recombination. However, comparisons based on the age of elements and their chromosomal location indicate that integration-site specificity is likely to be the primary factor determining distribution of the Athila and Tat sublineages of the Metaviridae. We predict that, like retroelements in yeast, the Athila and Tat elements target integration to pericentromeric regions by recognizing a specific feature of pericentromeric heterochromatin.

Arabidopsis↗

PubMed2006

Distribution of Ty3-gypsy- and Ty1-copia-like DNA sequences in the genus Helianthus and other Asteraceae.

Two repeated DNA sequences, pHaS13 and pHaS211, which revealed similarity to the int gene of Ty3-gypsy retrotransposons and the RNAse-H gene of Ty1-copia retroelements, respectively, were surveyed in Asteraceae species and within the genus Helianthus. Southern analysis of the genome of selected Asteraceae that belong to different tribes showed that pHaS13- and pHaS211-related subfamilies of gypsy- and copia-like retroelements are highly redundant only in Helianthus and, to a lesser extent, in Tithonia, a Helianthus strict relative. However, under low stringency posthybridization washes, bands were observed in almost all the other Asteraceae tested when pHaS13 was used as a probe, and in several species when pHaS211 was hybridized. FISH analysis of pHaS13 or pHaS211 probes was performed in species in which labelling was observed in Southern hybridizations carried out under high stringency conditions (Helianthus annuus, Tithonia rotundifolia, Ageratum spp., Leontopodium spp., Senecio vulgaris for pHaS13, and H. annuus, Tithonia rotundifolia, and S. vulgaris for pHaS211). Scattered labelling was observed over all metaphase chromosomes, indicating a large dispersal of both Ty3-gypsy- and Ty1-copia-like retroelements. However, preferential localization of Ty3-gypsy-like sequences at centromeric chromosome regions was observed in all of the species studies but one, even in species in which pHaS13-related elements are poorly represented. Ty1-copia-like sequences showed preferential localization at the chromosome ends only in H. annuus. To study the evolution of gypsy- and copia-like retrotransposons in Helianthus, cladograms were built based on the Southern blot hybridization patterns of pHaS13 or pHaS211 sequences to DNA digests of several species of this genus. Both cladograms agree in splitting the genomes studied into annuals and perennials. Differences that occurred within the clades of perennial and annual species between gypsy- and copia-like retroelements indicated that these retrotransposons were differentially active during Helianthus speciation, suggesting that the evolution of the 2 retroelement families was, within limits, independent.

Amino Acid Sequence↗

PubMed1990

Integration specificity of retrotransposons and retroviruses.

Analysis of in vivo integration patterns has provided no data to support the notion that retroelement integration is random. Rather, the diversity of insertion patterns of retroelements suggests numerous ways in which genomic DNA is identified for preferential targeting. These range from specific to general and include sequence content, removal of chromatin proteins, nuclear localization, distinctive topology, and association with particular trans-acting factors. Many are similar to mechanisms already demonstrated to affect activities of previously described recombinases. Moreover, such proposed targeting mechanisms could act directly or indirectly to influence integration site selection. A variety of observations are consistent with the preferential use of open chromatin for retroelement insertion. The site-specific retroelements insert into transcribed regions. In vitro studies with retroviruses and Ty1 have shown that naked DNA can function, at least under some conditions, as a target. The association of integration sites of retroviruses and regions in which DNase I hypersensitive sites exist and the preferential integration of Ty1 at the 5' ends of some genes might suggest that regions which do not have phased nucleosomes are targets for integration. Is targeting to transcriptionally active regions essentially passive, because they are not densely associated with chromatin proteins, or is targeting active in the sense of being a more specific process? Specific targets could be generated from DNA or protein motifs. Nucleosome-free regions are associated with a variety of nonnucleosome proteins, including topoisomerases, nuclear matrix proteins, transcription factors, or replication proteins. These then are candidates for proteins which target integration directly, by associating with the transposition complex or, indirectly, by inducing changes in the DNA. Polymerase III-transcribed genes, which are relatively defined targets of integration for some retrotransposon systems, probably exemplify several of these mechanisms. Promoter sequences may be directly involved in targeting some elements and positioning of the transcription complex may fix the integration sites of others. The most common sequence feature of characterized in vivo insertion sites is that they are AT-rich. This may reflect specificity of the IN protein, particularly the gypsylike elements, increased nicking of DNA, which is relatively weakly base-paired, as appears to be the case for the FLP recombinase (130), or simply the AT content of accessible regions in chromatin. Some of these questions will be resolved by the characterization of in vitro integration sites that have been recovered by physical means, rather than by biological assay. The insertion patterns of a number of retroposons suggest that retroelements can insert with a high degree of sequence specificity.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed2006

Retrotransposon sequence variation in four asexual plant species.

Transposable elements (TEs) can be viewed as genetic parasites that persist in populations due to their capacity for increase in copy number and the inefficacy of selection against them. A corollary of this hypothesis is that TEs are more likely to spread within sexual populations and be eliminated or inactivated within asexual populations. While previous work with animals has shown that asexual taxa may contain less TE diversity than sexual taxa, comparable work with plants has been lacking. Here we report the results of a study of Ty1/copia, Ty3/gypsy, and LINE-like retroelement diversity in four asexual plant species. Retroelement-like sequences, with a high degree of conservation both within and between species, were isolated from all four species. The sequences correspond to several previously annotated retroelement subfamilies. They also exhibit a pattern of nucleotide substitution characterized by an excess of synonymous substitutions, suggestive of a history of purifying selection. These findings were compared with retroelement sequence evolution in sexual plant taxa. One likely explanation for the discovery of conserved TE sequences in the genomes of these asexual taxa is simply that asexuality within these taxa evolved relatively recently, such that the loss and breakdown of TEs is not yet detectable through analysis of sequence diversity. This explanation is examined by conducting stochastic simulation of TE evolution and by using published information to infer rough estimates of the ages of asexual taxa.

Amino Acid Sequence↗

PubMed2000

Identification and chromosomal localization of the monkey retrotransposon in Musa sp.

Retroelements are ubiquitous features of eukaryotic genomes, often accounting for a substantial fraction of their total DNA content. One major group of retroelements, which includes the gypsy and copia-like elements, is distinguished by the presence of long terminal repeats (LTRs). We have identified and partially characterized a sequence from banana (Musa acuminata cv. Grand Nain) which shows significant homology to gypsy-like LTR retroelements from other species. The element, named monkey, shows a high degree of homology to the reverse transcriptase, RNase H and integrase genes of retroelements from plants, fungi and yeast. However, several stop codons are present in the major ORF of this element, suggesting that this copy of monkey, if functional, is non-autonomous. Southern analysis indicated that monkey is present in both the A and B genomes of Musa, and that it is found in 200-500 copies per haploid genome in cv. Grand Nain. Chromosomal localization by fluorescent in-situ hybridization indicates that copies of monkey are concentrated in the nucleolar organizer regions and colocalize with rRNA genes. Other copies of monkey appear to be dispersed throughout the genome.

Chromosome Mapping↗

PubMed2006

APOBEC3A is a potent inhibitor of adeno-associated virus and retrotransposons.

APOBEC3 proteins constitute a family of cytidine deaminases that provide intracellular resistance to retrovirus replication and transposition of endogenous retroelements. One family member, APOBEC3A (hA3A), is an orphan, without any known antiviral activity. We show that hA3A is catalytically active and that it, but none of the other family members, potently inhibits replication of the parvovirus adeno-associated virus (AAV). hA3A was also a potent inhibitor of the endogenous LTR retroelements, MusD, IAP, and the non-LTR retroelement, LINE-1. Its function was dependent on the conserved amino acids of the hA3A active site, consistent with a role for cytidine deamination, although mutations in retroelement sequences were not found. These findings demonstrate the potent activity of hA3A, an APOBEC3 family member with no previously identified function. They also highlight the functional differences between APOBEC3 proteins. The APOBEC3 family members have distinct functions and may have evolved to resist various classes of genetic elements.

Cell Line, Tumor↗

PubMedDate not supplied

Identification of non-autonomous non-LTR retrotransposons in the genome of Trypanosoma cruzi.

As observed for most eukaryotic cells, trypanosomatids contains non-LTR retrotransposons randomly inserted in the nuclear genome. Autonomous retroelements which, code for their own transposition, have been characterized in Trypanosoma brucei (ingi) and Trypanosoma cruzi (L1Tc), whereas non-autonomous retroelements have only been characterized in T. brucei (RIME). Here, we have characterized in the genome of Trypanosoma cruzi four complete copies of a non-autonomous non-LTR retrotransposon, called NARTc. This 0.26 kb NARTc element has the characteristics of non-LTR retrotransposons: the presence a poly(dA) tail and of a short flanking duplicated motif. Analysis of the Genome Survey Sequence databases indicated that the Trypanosoma cruzi haploid genome contains about 140 NARTc copies and about twice as many L1Tc copies. Interestingly, the NARTc and L1Tc retroelements share, with the Trypanosoma brucei ingi and RIME retrotransposons, a common sequence (the first 45 bp with 91% identity), whereas the remaining sequences are very divergent. This suggests that these four trypanosome non-LTR retrotransposons were derived from the same common ancester and the sequence of their 5'-extremity may have a functional role. In addition, the genome of Leishmania major contains the same conserved motif present in the trypanosome retroelements, whicle no transposable elements have been detected so far in Leishmania sp.

Animals↗

PubMed1999

Moose, a new family of LTR-retrotransposons in the mosquito Anopheles gambiae.

A novel LTR retrotransposable element called moose has been cloned and characterized from the malaria transmitting mosquito, Anopheles gambiae. This element has all the characteristic features of LTR retroelements and is related to retroelements from other insects and nematodes, belonging to a subgroup of retroelements distinct from the copia/Ty1 and gypsy/Ty3 groups. The moose element appears to be active in A. gambiae, and strong RNA expression is detected in the male and female gonads. The use of this retroelement as a potential vector for germ line transformation is discussed.

Amino Acid Sequence↗

PubMed2004

Structural features of the rice chromosome 4 centromere.

A complete sequence of a chromosome centromere is necessary for fully understanding centromere function. We reported the sequence structures of the first complete rice chromosome centromere through sequencing a large insert bacterial artificial chromosome clone-based contig, which covered the rice chromosome 4 centromere. Complete sequencing of the 124-kb rice chromosome 4 centromere revealed that it consisted of 18 tracts of 379 tandemly arrayed repeats known as CentO and a total of 19 centromeric retroelements (CRs) but no unique sequences were detected. Four tracts, composed of 65 CentO repeats, were located in the opposite orientation, and 18 CentO tracts were flanked by 19 retroelements. The CRs were classified into four types, and the type I retroelements appeared to be more specific to rice centromeres. The preferential insert of the CRs among CentO repeats indicated that the centromere-specific retroelements may contribute to centromere expansion during evolution. The presence of three intact retrotransposons in the centromere suggests that they may be responsible for functional centromere initiation through a transcription-mediated mechanism.

Base Sequence↗

PubMed2005

Whole-genome experimental identification of insertion/deletion polymorphisms of interspersed repeats by a new general approach.

A new experimental technique for genome-wide detection of integration sites of polymorphic retroelements (REs) is described. The technique allows one to reveal the absence of a retroelement in an individual genome provided that this retroelement is present in at least one of several other genomes under comparison. Since quite a number of genomes are compared simultaneously, the search for polymorphic REs insertions is very efficient. The technique includes two whole-genome selective PCR amplifications of sequences flanking REs: one for a particular genome and another one for a mixture of ten different genomes. A subsequent subtractive hybridization of the obtained amplicons with DNA of a particular genome as driver results in isolation of polymorphic insertions. The technique was successfully applied for identification of 41 new polymorphic human AluYa5/Ya8 insertions. Among them, 18 individual Alu elements first sequenced in this work were not found in the available human genome databases. This result suggests that significant part of polymorphic REs were not identified during genome sequencing and remain to be detected and characterized. The proposed method does not depend on preliminary knowledge of evolutionary history of retroelements and can be applied for identification of insertion/deletion polymorphic markers in genomes of different species.

Alu Elements↗

PubMed2000

A contiguous 66-kb barley DNA sequence provides evidence for reversible genome expansion.

Organisms with large genomes contain vast amounts of repetitive DNA sequences, much of which is composed of retrotransposons. Amplification of retrotransposons has been postulated to be a major mechanism increasing genome size and leading to "genomic obesity." To gain insights into the relation between retrotransposons and genome expansion in a large genome, we have studied a 66-kb contiguous sequence at the Rar1 locus of barley in detail. Three genes were identified in the 66-kb contig, clustered within an interval of 18 kb. Inspection of sequences flanking the gene space unveiled four novel retroelements, designated Nikita, Sukkula, Sabrina, and BAGY-2 and several units of the known BARE-1 element. The retroelements identified are responsible for at least 15 integration events, predominantly arranged as multiple nested insertions. Strikingly, most of the retroelements exist as solo LTRs (Long Terminal Repeats), indicating that unequal crossing over and/or intrachromosomal recombination between LTRs is a common feature in barley. Our data suggest that intraelement recombination events deleted most of the original retrotransposon sequences, thereby providing a possible mechanism to counteract retroelement-driven genome expansion.

Base Composition↗

PubMed2005

Evolution of DNA sequence nonhomologies among maize inbreds.

Allelic chromosomal regions totaling more than 2.8 Mb and located on maize (Zea mays) chromosomes 1L, 2S, 7L, and 9S have been sequenced and compared over distances of 100 to 350 kb between the two maize inbred lines Mo17 and B73. The alleles contain extended regions of nonhomology. On average, more than 50% of the compared sequence is noncolinear, mainly because of the insertion of large numbers of long terminal repeat (LTR)-retrotransposons. Only 27 LTR-retroelements are shared between alleles, whereas 62 are allele specific. The insertion of LTR-retrotransposons into the maize genome is statistically more recent for nonshared than shared ones. Most surprisingly, more than one-third of the genes (27/72) are absent in one of the inbreds at the loci examined. Such nonshared genes usually appear to be truncated and form clusters in which they are oriented in the same direction. However, the nonshared genome segments are gene-poor, relative to regions shared by both inbreds, with up to 12-fold difference in gene density. By contrast, miniature inverted terminal repeats (MITEs) occur at a similar frequency in the shared and nonshared fractions. Many times, MITES are present in an identical position in both LTRs of a retroelement, indicating that their insertion occurred before the replication of the retroelement in question. Maize ESTs and/or maize massively parallel signature sequencing tags were identified for the majority of the nonshared genes or homologs of them. In contrast with shared genes, which are usually conserved in gene order and location relative to rice (Oryza sativa), nonshared genes violate the maize colinearity with rice. Based on this, insertion by a yet unknown mechanism, rather than deletion events, seems to be the origin of the nonshared genes. The intergenic space between conserved genes is enlarged up to sixfold in maize compared with rice. Frequently, retroelement insertions create a different sequence environment adjacent to conserved genes.

Alleles↗

PubMed2005

Three distinct chromatin domains in telomere ends of polytene chromosomes in Drosophila melanogaster Tel mutants.

Drosophila melanogaster telomeric DNA is known to comprise two domains: the terminal tract of retrotransposons (HeT-A, TART and TAHRE) and telomere-associated sequences (TAS). Chromosome tips are capped by a protein complex, which is assembled on the chromosome ends independently of the underlying terminal DNA sequences. To investigate the properties of these domains in salivary gland polytene chromosomes, we made use of Tel mutants. Telomeres in this background are elongated owing to the amplification of a block of terminal retroelements. Supercompact heterochromatin is absent from the telomeres of polytene chromosomes: electron microscopy analysis identifies the telomeric cap and the tract of retroelements as a reticular material, having no discernible banding pattern, whereas TAS repeats appear as faint bands. According to the pattern of bound proteins, the cap, tract of retroelements and TAS constitute distinct and non-overlapping domains in telomeres. SUUR, HP2, SU(VAR)3-7 and H3Me3K27 localize to the cap region, as has been demonstrated for HP1. All these proteins are also found in pericentric heterochromatin. The tract of retroelements is associated with proteins characteristic for both heterochromatin (H3Me3K9) and euchromatin (H3Me3K4, JIL-1, Z4). The TAS region is enriched for H3Me3K27. PC and E(Z) are detected both in TAS and many intercalary heterochromatin regions. Telomeres complete replication earlier than heterochromatic regions. The frequency of telomeric associations in salivary gland polytene chromosomes does not depend on the SuUR gene dosage, rather it appears to be defined by the telomere length.

Animals↗

PubMed1996

Synthesis and analysis of a 640-bp pol region of novel human endogenous retroviral sequences and their evolutionary relationships.

Nucleotide sequence comparisons of the pol gene among 47 retroelements identified two very conserved regions, separated by a span of approximately 640 bp, that have not been previously reported. A set of mixed oligonucleotide primers, 5'-MOP-2 and 3'-MOP-2, homologous to these two conserved pol regions was constructed for use in detection of retroelements. When MOPs-2 were employed in PCR amplification studies, products of about 0.64 kb in size were amplified from human and mouse genomic DNAs and from HIV-1 proviral DNA, but not from negative control plasmid DNAs. The PCR products amplified with MOPs-2 from human LuC-1 teratocarcinoma cell DNA were subcloned and sequenced. Five clones of approximately 0.64 kb in size were identified, and sequence comparisons with all entries in GenBank indicated that these five clones have highest homology, in a range of 64.31 to 98.65%, with the corresponding pol region of HERV-K10 and HM-16 of the human endogenous retrovirus-K (HERV-K) family. Southern hybridizations at high stringency demonstrated that these five clones are present in all human DNAs tested. The evolutionary relationships of these clones with the equivalent pol region of other retroelements were defined by phylogenetic analyses that placed three clones into the HERV-K family and two clones into a new family of human endogenous retroelements. In addition, clone HERV-(K)73 contains the smaller PV1b pol segment that was reported to be selectively expressed in blood leukocytes of patients with polycythemia vera.

Amino Acid Sequence↗

PubMed2006

High-molecular-mass APOBEC3G complexes restrict Alu retrotransposition.

APOBEC3G (A3G) and related deoxycytidine deaminases are potent intrinsic antiretroviral factors. A3G is expressed either as an enzymatically active low-molecular-mass (LMM) form or as an enzymatically inactive high-molecular-mass (HMM) ribonucleoprotein complex. Resting CD4 T cells exclusively express LMM A3G, where it functions as a powerful postentry restriction factor for HIV-1. Activation of CD4 T cells promotes the recruitment of LMM A3G into 5- to 15-MDa HMM complexes whose function is unknown. Using tandem affinity purification techniques coupled with MS, we identified Staufen-containing RNA-transporting granules and Ro ribonucleoprotein complexes as specific components of HMM A3G complexes. Analysis of RNAs in these complexes revealed Alu and small Y RNAs, two of the most prominent nonautonomous mobile genetic elements in human cells. These retroelement RNAs are recruited into Staufen-containing RNA-transporting granules in the presence of A3G. Retrotransposition of Alu and hY RNAs depends on the reverse transcriptase machinery provided by long interspersed nucleotide elements 1 (L1). We now show that A3G greatly inhibits L1-dependent retrotransposition of marked Alu retroelements not by inhibiting L1 function but by sequestering Alu RNAs in cytoplasmic HMM A3G complexes away from the nuclear L1 enzymatic machinery. These findings identify nonautonomous Alu and hY retroelements as natural cellular targets of A3G and highlight how different forms of A3G uniquely protect cells from the threats posed by exogenous retroviruses (LMM A3G) and endogenous retroelements (HMM A3G).

APOBEC-3G Deaminase↗