Biomedical subjects

M A Grandbastien

Publications and source records attributed to M A Grandbastien.

At least 19 recordsLinked to original sources

PubMed2001

Retrolyc1 subfamilies defined by different U3 LTR regulatory regions in the Lycopersicon genus.

Retrolycl, a Ty1/copia-like element, was originally isolated from the Lycopersicon peruvianum genome and shown to be present also in other Lycopersicon species. It shares extensive similarities with Tntl, except in its U3 regulatory region. In order to evaluate Retrolycl diversity, we analyzed partial sequences including both coding domains and the U3 regulatory region in four different species of the Lycopersicon genus. Two Retrolycl subfamilies defined by different U3 regions were identified. RetrolyclA is most abundant in L. peruvianum and L. hirsutum, while Retrolyc1B is distributed in all four species studied here. The RetrolyclA U3 region contains tandemly repeated elements of 53 bp. Transient expression analysis suggests that Retrolyc1A is a transcriptionally active family, and that the repeated motifs found in its U3 region are important transcriptional regulatory elements.

Base Sequence↗

PubMed2001

Retrotransposons of the Tnt1B family are mobile in Nicotiana plumbaginifolia and can induce alternative splicing of the host gene upon insertion.

Active retrotransposons have been identified in Nicotiana plumbaginifolia by their ability to disrupt the nitrate reductase gene in chlorate-resistant mutants selected from protoplast-derived cultures. In mutants E23 and F97, two independent insertions of Tnp2, a new retrotransposon closely related to the tobacco Tnt1 elements, were detected in the nitrate reductase gene. These two Tnp2 elements are members of the Tnt1B subfamily which shows that Tnt1B elements can be active and mutagenic in the N. plumbaginifolia genome. Furthermore, these results suggest that Tnt1B is the most active family of Tntl elements in N. plumbaginifolia, whereas in tobacco only members of the Tnt1A subfamily were found inserted in the nitrate reductase gene. The transcriptional regulations of Tnp2 and Tnt1A elements are most probably different due to non-conserved U3 regions. Our results thus support the hypothesis that different Nicotiana species contain different active Tntl subfamilies and that only one active Tntl subfamily might be maintained in each of these species. The Tnp2 insertion found in the F97 mutant was found to be spliced out of the nitrate reductase mRNA by activation of cryptic donor and acceptor sites in the nitrate reductase and the Tnp2 sequences respectively.

Alternative Splicing↗

PubMed2001

The mobility of the tobacco Tnt1 retrotransposon correlates with its transcriptional activation by fungal factors.

We have analyzed the stress-induced amplification of the tobacco Tnt1 element, one of the rare active plant retrotransposons. Tnt1 mobility was monitored using the retrotransposon-anchored SSAP strategy that allows the screening of multiple insertion sites of high copy number elements. We have screened for Tnt1 insertion polymorphisms in plants regenerated from mesophyll leaf cells, either via explant culture or via protoplast isolation. The second procedure includes an overnight exposure to fungal extracts known to induce high levels of Tnt1 transcription. Newly transposed Tnt1 copies were detected in nearly 25% of the plants regenerated via protoplast isolation, and in less than 3% of the plants derived from explant culture. These results show that Tnt1 transcription is followed by transposition, and that fungal extracts efficiently activate Tnt1 mobility. Transcription appears to be the key step to controlling Tnt1 amplification, as newly transposed Tnt1 copies show high sequence similarities to the subpopulations of transcribed Tnt1 elements. Our results provide direct evidence that factors of microbial origin are able to induce retrotransposon amplification in plants, and strengthen the hypothesis that stress modulation of transposable elements might play a role in generating host genetic plasticity in response to environmental stresses.

Base Sequence↗

PubMed2001

Three Tnt1 subfamilies show different stress-associated patterns of expression in tobacco. Consequences for retrotransposon control and evolution in plants.

The genomes of most Nicotiana species contain three different subfamilies of the Tnt1 retrotransposon, which differ completely in their U3 sequence, whereas the rest of the sequence is relatively constant. The results presented here show that all three Tnt1 subfamilies are expressed in tobacco (Nicotiana tabacum) and that the U3 sequence variability correlates with differences in the pattern of expression of the Tnt1 elements. Each of the three Tnt1 subfamilies is induced by stress, but their promoters have a different response to different stress-associated signaling molecules. The Tnt1A subfamily is particularly strongly induced by elicitors and methyl jasmonate, whereas expression of the Tnt1C subfamily is more sensitive to salicylic acid and auxins. The direct relationship between U3 sequence variability and differences in the stress-associated expression of the Tnt1 elements present in a single host species gives support to our model that postulates that retrotransposons have adapted to their host genomes through the evolution of highly regulated promoters that mimic those of the stress-induced plant genes. Moreover, here we show that the analysis of the transcriptional control of a retrotransposon population such as Tnt1 provides new insights into the study of the complex and still poorly understood network of defense- and stress-induced plant signal transduction pathways.

2,4-Dichlorophenoxyacetic Acid↗

PubMed1999

Retrolyc1-1, a member of the Tntl retrotransposon super-family in the Lycopersicon peruvianum genome.

Retrotransposons are ubiquitous mobile genetic elements that transpose through an RNA intermediate. One of the best known plant retrotransposon, Tnt1, was isolated from tobacco and showed an extensive distribution in the Nicotiana genus. We investigated the presence of related sequences in the Lycopersicon genus, another member of the Solanaceae family. Hybridization experiments performed using Tnt1 probes indicated that homologous sequences were present in all Lycopersicon species, indicating that these Tnt1-related sequences, that we named Retrolyc1, are distributed throughout the Lycopersicon genus. Different distribution patterns were detected between species, demonstrating a potential use of Retrolyc1 elements as molecular markers. An incomplete Retrolyc1 sequence, that we named Retrolyc1-1, was isolated from an L. peruvianum genomic library. Retrolyc1-1 shows extensive homology with Tnt1 sequences except in the LTR U3 region. Since this region is known to be involved in the control of transcription, this strongly suggests the existence of different patterns of regulation for Tnt1 and Retrolyc1 elements. The study of these two elements within the Solanaceae family may provide interesting models for retrotransposon evolution within this group and transmission in host genomes.

Journal Article↗

PubMed1999

Retrolycl-1, a member of the tntl retrotransposon super-family in the Lycopersicon peruvianum genome.

Base Sequence↗

PubMed1998

The evolutionary analysis of the Tnt1 retrotransposon in Nicotiana species reveals the high variability of its regulatory sequences.

We studied the evolution of the tobacco Tnt1 retrotransposon by analyzing Tnt1 partial sequences containing both coding domains and U3 regulatory sequences obtained from a number of Nicotiana species. We detected three different subfamilies of Tnt1 elements, Tnt1A, Tnt1B, and Tnt1C, that differ completely in their U3 regions but share conserved flanking coding and LTR regions. U3 divergence between the three subfamilies is found in the region that contains the regulatory sequences that control the expression of the well-characterized Tnt1-94 element. This suggests that expression of the three Tnt1 subfamilies might be differently regulated. The three Tnt1 subfamilies were present in the Nicotiana genome at the time of species divergence, but have evolved independently since then in the different genomes. Each Tnt1 subfamily seems to have conserved its ability to transpose in a limited and different number of Nicotiana species. Our results illustrate the high variability of Tnt1 regulatory sequences. We propose that this high sequence variability could allow these elements to evolve regulatory mechanisms in order to optimize their coexistence with their host genome.

Base Sequence↗

PubMed1997

The promoter of the tobacco Tnt1 retrotransposon is induced by wounding and by abiotic stress.

The transcription of the tobacco Tnt1 retrotransposon was previously shown to be induced, in tobacco and in heterologous species, by microbial elicitors and by pathogen infections. We report here that the expression of the Tnt1 promoter is also activated in heterologous species such as tomato and Arabidopsis by wounding, freezing and by other abiotic factors known to induce the plant defence response, such as salicylic acid, CuCl2, or oxidative stress. A similar regulation is observed in tobacco for most treatments. The induction of the Tnt1 promoter expression by wounding remains localized around injury points. In CuCl2-treated Arabidopsis plants, the transcription of Tnt1 is correlated with accumulation of the phytoalexin camalexin and with the expression of the EL13 defence gene. The interest of the Tnt1 promoter as a sensitive indicator of the plant defence responses is discussed.

Arabidopsis↗

PubMed1997

In vivo characterization of transcriptional regulatory sequences involved in the defence-associated expression of the tobacco retrotransposon Tnt1.

The expression of the tobacco retrotransposon Tnt1 is induced by wounding, pathogen infections as well as microbial elicitors and abiotic factors known to induce the plant defence response. We report here that the LTR U3 region is sufficient to mediate transcriptional activation by biotic and abiotic elicitors in stable transgenic conditions. We have used in vivo footprinting techniques in order to analyse the cis-regulatory elements of the LTR U3 region that mediate the induction of Tnt1 expression. Our results indicate that a tandemly repeated short element, named BII box, is involved in the transcriptional activation of the tobacco retrotransposon Tnt1 in association with the plant defence signaling cascade.

Algal Proteins↗

PubMed1997

Quasispecies in retrotransposons: a role for sequence variability in Tnt1 evolution.

Retroviral replication is a very error-prone process. Replication of retroviruses gives rise to populations of closely related but different genomes referred to as 'quasispecies'. This huge swarm of different sequences constitutes a reservoir of potentially useful genomes in case of an environmental change, endowing retroviruses with extreme adaptability. Retrotransposons are mobile genetic elements closely related to retroviruses, and retrotransposition is as error prone as retroviral replication. The Tnt1 retrotransposon is present in hundreds of copies in the genome of tobacco that show a high level of sequence heterogeneity. When Tnt1 is expressed, its RNA is not a single sequence but a population of sequences displaying a quasispecies-like structure. This population structure gives to Tnt1, as in the case of retroviruses, a high sequence plasticity and an adaptive capacity. We propose this adaptivity as the major reason for Tnt1 maintenance in Nicotiana genomes and we discuss in this paper the importance of sequence variability for Tnt1 evolution.

Adaptation, Physiological↗

PubMed1997

The expression of the tobacco Tnt1 retrotransposon is linked to plant defense responses.

Activation of retrotransposons by stresses and external changes is common in all eukaryotic systems, including plants. The transcription of the tobacco Tnt1 retrotransposon was studied in its natural host as well as in Arabidopsis and tomato. It is activated by factors of microbial origin, by external stresses, and by viral, bacterial, and fungal attacks. Tnt1 expression is linked with the biological responses of the plant to the elicitor or to the pathogen attack and in particular with the early steps of the metabolic pathways leading to the activation of plant defense genes. In most cases, the basic features of Tnt1 regulation in tobacco are maintained in tomato and Arabidopsis, but some host-specific regulations were shown. The U3 region of the Tnt1 LTR contains the major cis-acting components of Tnt1 transcriptional activation in association with the plant defense responses. Furthermore, the Tnt1 U3 region, and especially the tandemly repeated BII boxes, contains several sequences similar to well-characterized motifs involved in the activation of several plant defense genes. The possible origin of Tnt1 regulatory sequences as well as the biological implications of Tnt1 activation by pathogen attacks are discussed.

Arabidopsis↗

PubMed1996

Molecular and functional characterization of Slide, an Ac-like autonomous transposable element from tobacco.

A new transposable element of tobacco, Slide, was isolated from the tl mutant line, which shows somatic instability, after its transposition into a locus encoding nitrate reductase (NR). The Slide-124 element is 3733 bp long and its coding sequences show similarities with conserved domains of the transposases of Ac, Tam3 and hobo. Excision from the NR locus is detectable in somatic leaf tissues and Slide mobility is triggered by in vitro tissue culture. Slide excision events create footprints similar to those left by Ac and Tam3. Tobacco lines derived from the tl mutant line seem characterized by unmethylated copies of a few members of the highly repetitive Slide family. Slide mobility was monitored in transient expression assays. In wild-type tobacco protoplasts, the complete Slide element, as well as a defective copy, is able to excise. The complete Slide element, but not the defective version, is able to excise in protoplasts of the heterologous species lettuce (Lactuca sativa). These results show that Slide carries the functions required for its own mobility, and represents the first autonomous Ac-like element characterized in Solanaceae species.

Amino Acid Sequence↗

PubMed1995

Sequence variability within the tobacco retrotransposon Tnt1 population.

Retroviruses consist of populations of different but closely related genomes referred to as quasispecies. A high mutation rate coupled with extremely rapid replication cycles allows these sequences to be highly interconnected in a rapid equilibrium. It is not known if other retroelements can show a similar population structure. We show here that when the tobacco Tnt1 retrotransposon is expressed, its RNA is not a unique sequence but a population of different but closely related sequences. Nevertheless, this highly variable population is not in a rapid equilibrium and could not be considered as a quasispecies. We have thus named the structure presented by Tnt1 RNA quasispecies-like. We show that the expression of Tnt1 in different situations gives rise to different populations of Tnt1 RNA sequences, suggesting an adaptive capacity for this element. The analysis of the variability within the total genomic population of Tnt1 elements shows that mutations frequently occur in important regulatory elements and that defective elements are often produced. We discuss the implications that this population structure could have for Tnt1 regulation and evolution.

Base Sequence↗

PubMed1995

RNA-mediated transposition of the tobacco retrotransposon Tnt1 in Arabidopsis thaliana.

The tobacco (Nicotiana tabacum) retrotransposon Tnt1 was introduced into Arabidopsis thaliana. In this heterologous host plant species, Tnt1 undergoes an RNA-mediated transposition and creates a 5 bp duplication at the insertion sites. This is the first report of transposition of a retrotransposon after introduction into a heterologous host species. Tnt1 transposed during in vitro regeneration of transformed A.thaliana, but no transposition event was detected as happening in T2 and T3 generation plants. Newly synthesized copies of Tnt1 can integrate into coding regions of the host DNA. Our results open up the possibility of using Tnt1 as a new tool for insertional mutagenesis and functional analysis of plant genomes, in addition to the strategies of T-DNA and transposon tagging.

Amino Acid Sequence↗

PubMed1994

Expression of the tobacco Tnt1 retrotransposon promoter in heterologous species.

The expression of the tobacco (Nicotiana tabacum) retrotransposon Tnt1 has previously been shown to be strongly regulated and driven from the 5' long terminal repeat (LTR). We report here that the Tnt1 LTR can promote activity of the beta-glucuronidase (GUS) reporter gene in two heterologous species of the Brassicaceae family, namely rapeseed (Brassica napus) and Arabidopsis thaliana. The translational LTR-GUS fusion was active in transient expression studies performed with tobacco and rapeseed protoplasts, indicating that the LTR sequences are recognized in heterologous species. Our results also showed that Tnt1 LTR-promoted GUS expression in transgenic Arabidopsis is strongly regulated, and that, in contrast to tobacco, hormonal activation plays a significant role in the expression of the Tnt1 LTR in Arabidopsis. LTR sequences were shown to be more effective than the CaMV 35S enhancer region in transient expression studies performed with tobacco or rapeseed protoplasts, and substitution of the LTR sequences upstream from the major transcriptional start with the CaMV 35S enhancer region gave high levels of expression in transgenic tobacco and Arabidopsis leaves, suggesting that a Tnt1 element with similar substitutions in its 5' LTR might be suited for gene-tagging experiments in heterologous species.

Arabidopsis↗

PubMed1994

Functional analysis of the tobacco Tnt1 retrotransposon.

Retroelements represent by far the largest and most widespread class of mobile genetic elements. Representative of several classes of retrotransposons have been characterized in a broad range of plant species, but only a few of them have been shown to be active. Among these, the tobacco Tnt1 retrotransposon has been isolated after insertion mutagenesis and is one of the very few to be transcriptionally active. Tnt1 expression is strongly regulated in a tissue-specific and developmental manner. Moreover, Tnt1 expression is induced by a range of biotic or abiotic elicitors, which all have in common the ability to induce the plant defense response. Regulatory sequences involved in this elicitor-mediated induction have been located in the LTR U3 region. The link between Tnt1 activation and the plant defense response might represent an example of the involvement of transposable elements in genome restructurations needed in response to environmental fluctuations such as pathogen attacks.

Base Sequence↗

PubMed1993

Characterisation of LTR sequences involved in the protoplast specific expression of the tobacco Tnt1 retrotransposon.

The tobacco Tnt1 retrotransposon is the only plant retrotransposon that has been shown to be transcriptionally active, and its transcription is strongly induced when preparing leaf-derived protoplasts. We have analysed in this paper the LTR sequences important for Tnt1 expression in tobacco protoplasts. We show that LTR sequences upstream of the TATA box are sufficient to confer protoplast-dependent induction to a heterologous promoter. We also show that this region contains two short activator elements, and that one of these sequences, BII, interacts with protoplast-specific nuclear factors.

Base Sequence↗

PubMed1992

Retroelements in higher plants.

Representatives of several classes of retroelements have been characterized in a broad range of plant species, where they appear at variable and sometimes very high copy numbers. So far, only a very small number of plant elements have been shown to be active, and this activity seems to be restricted to specific situations of 'genomic shock'. Although it is not yet known whether the presence of retroelements is linked to the high level of variability found in plant genomes, it is now clear that retrotransposons are ancient and ubiquitous components of plant genomes, and could play an important role in plant evolution.

Biological Evolution↗