Biomedical subjects

E S Coen

Publications and source records attributed to E S Coen.

33 records · Page 2Linked to original sources

PubMed1991

Allelic interactions at the nivea locus of Antirrhinum.

Most null alleles at the nivea (niv) locus are recessive to Niv+ and, when homozygous, give white flowers rather than the red of the wild type. In contrast, the niv-571 allele is semidominant; although it gives white flowers when homozygous, very pale flowers result when this allele is heterozygous with NIV+. We showed that in heterozygotes, niv-571 acts in trans to inhibit expression of its Niv+ homology 25-fold to 50-fold. The inhibition is reversible after meiosis and partially reversible somatically. The niv-571 allele carries a transposable element Tam3 insertion and three truncated copies of the niv gene, one copy being in inverse orientation. Analysis of two further niv alleles, niv-572 and niv-527, showed that excision of Tam3 from niv-571 does not affect the ability of the allele to repress Niv+ and that one truncated niv copy alone is insufficient to confer semidominance. The detailed structures of various semidominant niv alleles suggest that their effects in trans are not readily explained by production of antisense RNA but are more easily reconciled with a direct recognition/interaction between homologous genes, reminiscent of cosuppression and transvection phenomena described in other systems.

Acyltransferases↗

PubMed1991

Pigmentation mutants produced by transposon mutagenesis in Antirrhinum majus.

New pigmentation mutants were generated by transposon mutagenesis in Antirrhinum majus, in three previously described loci, nivea, delila and incolorata, and two new loci, daphne and olive. The wild-type olive gene is required for the production of dark-green leaves, and the daphne gene for the synthesis of flavones. Five out of the six mutants were both germinally and somatically unstable, indicating that they resulted from transposon insertions. Molecular analysis of the mutant at nivea (niv-600) showed that it was caused by insertion of a new transposon, Tam4. The sequence of Tam4 suggests that it is unable to transpose autonomously and that it is related to Tam1 and Tam2. All three of these transposons have identical inverted repeats, produce 3 bp target duplications, leave similar excision footprints and share at one end a 600-700 bp region containing many palindromic copies of a motif sequence, possibly required in cis for transposition. The somatic excision of Tam4 in niv-600 is at a very low rate compared to germinal excision but it can be activated by crossing to lines carrying derivative alleles of a Tam1 insertion at niv. Molecular analysis of four different pigmentation mutants has shown that insertions of Tam1, Tam2, Tam3 and Tam4 have been obtained, illustrating the potential of general transposon mutagenesis for trapping and isolating new transposons as well as for tagging genes.

Base Sequence↗

PubMed1990

floricaula: a homeotic gene required for flower development in antirrhinum majus.

Plants carrying the floricaula (flo) mutation cannot make the transition from inflorescence to floral meristems and have indeterminate shoots in place of flowers. The flo-613 allele carries a Tam3 transposon insertion, which allowed the isolation of the flo locus. The flo gene encodes a putative protein (FLO) containing a proline-rich N-terminus and a highly acidic region. In situ hybridization shows that the flo gene is transiently expressed in the very early stages of flower development. The earliest expression seen is in bract primordia, followed by sepal, petal, and carpel primordia, but no expression is detected in stamen primordia. This pattern of expression has implications for how flo affects phyllotaxis, organ identity, and determinacy. We propose that flo interacts in a sequential manner with other homeotic genes affecting floral organ identity.

Amino Acid Sequence↗

PubMed1990

Phenotypic effects of short-range and aberrant transposition in Antirrhinum majus.

We describe two novel ways in which changes in gene expression in Antirrhinum majus may arise as a consequence of the Tam3 transposition mechanism. One involves excision of Tam3 from the nivea gene promoter and insertion of two new Tam3 copies 3.4 kb and 2.1 kb away, on either side of the excision site. One of the new insertions is in the nivea coding region and completely blocks production of an active gene product. This allele probably arose by a symmetrical double transposition, following chromosome replication. The second case involves a small deletion at one end of Tam3 in the pallida gene, flanked by a sequence typical of a Tam3 excision footprint. This suggests that the end of Tam3 was cleaved at an early step in an attempted transposition and re-ligated back to its original flanking sequence. The alteration restores some expression to the pallida gene, suggesting that the ends of the intact Tam3 element contain components which can actively inhibit gene expression. The implications of these findings for the mechanism of Tam3 transposition and for the effects of Tam3 on host gene expression are discussed.

Alleles↗

PubMed1990

Floral homeotic mutations produced by transposon-mutagenesis in Antirrhinum majus.

To isolate and study genes controlling floral development, we have carried out a large-scale transposon-mutagenesis experiment in Antirrhinum majus. Ten independent floral homeotic mutations were obtained that could be divided into three classes, depending on whether they affect (1) the identity of organs within the same whorl; (2) the identity and sometimes also the number of whorls; and (3) the fate of the axillary meristem that normally gives rise to the flower. The classes of floral phenotypes suggest a model for the genetic control of primordium fate in which class 2 genes are proposed to act in overlapping pairs of adjacent whorls so that their combinations at different positions along the radius of the flower can specify the fate and number of whorls. These could interact with class 1 genes, which vary in their action along the vertical axis of the flower to generate bilateral symmetry. Both of these classes may be ultimately regulated by class 3 genes required for flower initiation. The similarity between some of the homeotic phenotypes with those of other species suggests that the mechanisms controlling whorl identity and number have been highly conserved in plant evolution. Many of the mutations obtained show somatic and germinal instability characteristic of transposon insertions, allowing the cell-autonomy of floral homeotic genes to be tested for the first time. In addition, we show that the deficiens (def) gene (class 2) acts throughout organ development, but its action may be different at various developmental stages, accounting for the intermediate phenotypes conferred by certain def alleles. Expression of def early in development is not necessary for its later expression, indicating that other genes act throughout the development of specific organs to maintain def expression. Direct evidence that the mutations obtained were caused by transposons came from molecular analysis of leaf or flower pigmentation mutants, indicating that isolation of the homeotic genes should now be possible.

DNA Transposable Elements↗

PubMed1990

Identification and genetic regulation of the chalcone synthase multigene family in pea.

Chalcone synthase (CHS) is a key enzyme in the biosynthesis of diverse flavonoids involved in disease resistance, nodulation, and pigmentation in pea. We describe a multigene family encoding CHS and the effects of two regulatory loci, a and a2, on the pattern of expression of three of its member genes. Two of the genes, CHS1 and CHS3, are expressed in both petal and root tissue, whereas expression of a third gene, CHS2, is detected only in roots. The products encoded by the a and a2 loci are required for the expression of the CHS1 gene and for wild-type levels of expression of the CHS3 gene in petal tissue. In root tissue, all three CHS genes are expressed and induced by CuCl2 regardless of the genotype at the a and a2 loci. These results show that the various members of the CHS multigene family interact in diverse ways with multiple genetic signals in the plant, providing a basis for the differential expression of these genes. Spatially specific genetic regulation of distinct members of a multigene family has been clearly demonstrated.

Acyltransferases↗

PubMed1989

A chromosome rearrangement suggests that donor and recipient sites are associated during Tam3 transposition in Antirrhinum majus.

We describe the structure of a chromosome rearrangement which changes the spatial pattern of expression of the pallida gene of Antirrhinum majus. The rearrangement involves a chromosome inversion of ~6 map units with one breakpoint at the end of a copy of the transposable element Tam3 located in the promoter region of the pallida locus. The sequence at the breakpoints shows that 5-7 bp, present once in the progenitor, has been duplicated and flanks both ends of the inversion. We propose that this structure arose from an aberrant Tam3 transposition, suggesting a model for normal transposition which involves physical association between donor and recipient sites. This may explain why transposition of plant transposable elements such as Ac in maize occurs preferentially to recipient sites closely linked to the donor site. Excision of the Tam3 copy located at the end of the chromosome inversion, results in a unique spatial pattern of pallida gene expression as a consequence of replacing all sequence 70 bp upstream of transcription by a new sequence. This pattern may be the result of deleting specific upstream components which regulate pallida expression and/or of changing the relative chromosome position of the pallida gene.

Journal Article↗

PubMed1989

Genetic interactions underlying flower color patterns in Antirrhinum majus.

Diverse spatial patterns of flower color in Antirrhinum can be produced by a series of alleles of pallida, a gene encoding an enzyme required for pigment biosynthesis. The alleles arose by imprecise excision of a transposable element, Tam3, and we show that they carry a series of deletions involving progressive removal of sequences adjacent to the excision site. This has enabled us to define three cis-acting upstream regions, A, B, and C, which differentially affect the level of pallida expression in distinct areas of the flower. We show further that an unlinked locus, delila, regulates the spatial distribution of pallida transcript. Deletion of regions ABC at the pallida locus uncouples pallida from regulation by delila, whereas deletion of A or AB brings pallida under regulation by delila in a new area of the flower. These results suggest that diverse patterns of pallida expression reflect the different ways in which alleles interact with a prepattern of both common and spatially specific genetic signals in the flower.

Alleles↗

PubMed1988

A semi-dominant allele, niv-525, acts in trans to inhibit expression of its wild-type homologue in Antirrhinum majus.

Niv-525 is a semi-dominant allele of the nivea locus, which encodes the enzyme chalcone synthase required for flower pigment biosynthesis in Antirrhinum majus. Plants heterozygous for niv-525 and wild-type (Niv+) allele, have flowers with a reduced intensity and novel spatial pattern of pigmentation compared with Niv+ homozygotes. In heterozygotes, niv-525 acts in trans to reduce the steady-state level of nivea transcript produced by its Niv+ homologue and hence the quantity of chalcone synthase protein. Niv-525 carries an inverted duplication of 207 bp in its promoter region which has arisen following excision of the transposable element Tam 3. This structure can be explained by a model of plant transposable element excision that involves resolution of two hairpin DNA molecules. Possible mechanisms for the trans-acting effect of niv-525 and its relationship to other examples of allelic interactions, such as transvection in Drosophila melanogaster, are discussed.

Acyltransferases↗

PubMed1986

Transposable elements generate novel spatial patterns of gene expression in Antirrhinum majus.

The pallida gene of A. majus encodes a product required for the synthesis of red flower pigment. We have shown that the unstable pallida(recurrens) mutation is due to the insertion of the Tam3 transposable element near the promoter of the gene. Imprecise excision of Tam3 alters pallida gene expression and generates new spatial patterns or different intensities of flower pigmentation. Distinct spatial patterns may also result from rearrangements induced by Tam3 that alter the relative position of the pallida gene. Changes in Tam3 structure or position result in new unstable phenotypes. These findings suggest that genes may be rendered genetically hypervariable as a consequence of transposable element insertion and excision.

Alleles↗

PubMed1985

Molecular analysis of instability in flower pigmentation of Antirrhinum majus, following isolation of the pallida locus by transposon tagging.

The pal locus of Antirrhinum majus was cloned using the transposable element, Tam 3, as a probe. The pal clone was used to examine, at the molecular level, those aspects of instability previously observed phenotypically and genetically. The effects of temperature and of genetic background on excision of the element at pal are considered, and related quantitatively to the phenotype. We describe the identification of the transcript of the pal locus and show that insertion of Tam 3 blocks the production of a normal pal transcript in developing flower buds.

Journal Article↗

PubMed1983

Unequal exchanges and the coevolution of X and Y rDNA arrays in Drosophila melanogaster.

We have examined the molecular basis of the response of individuals of D. melanogaster to artificial selection for high and low abdominal bristles. By monitoring the fate of particular rDNA spacer length variants associated with individually isolated X and Y chromosomes, we show that flies from the low bristle number selection lines have undergone an unequal exchange between the X and Y rDNA arrays. Such exchanges result in translocations between X and Y chromosomes, visualised as X.Y compound chromosomes at mitosis. Transfer of few copies of a length variant between X and Y indicates a clustering of variants. Flies that have reverted back to wild-type seemingly have undergone a second unequal exchange, giving rise to a compound X.Y chromosome containing Y rDNA of normal amounts. Unequal exchanges between X and Y rDNA arrays could contribute to the observed coevolution of rDNA sequences on these chromosomes. The biological significance of this outcome is discussed.

Animals↗

PubMed1982

Molecular drive.

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Animals↗

PubMed1982

Multiple Pol I initiation sequences in rDNA spacers of Drosophila melanogaster.

Sequence analysis of the rDNA nontranscribed spacer (NTS) of D.melanogaster shows that it contains many regions that are perfectly homologous with the Pol I transcription initiation site. Each initiation site homology is contained within a 240 base pair repeated sequence which are organised in a tandem array terminating 140 bases pairs upstream from the transcription initiation site. We discuss the significance of these regions and the maintenance of multiple homologies against the observed high rate of spacer divergence between related Drosophila species. In addition there are two other regions of repetition in the NTS: a 90 base pair periodicity commencing 16 bases downstream from the 3' end of the 28S gene and a number of 340 base pair repeats. A similar complex of different periodicities has been described in Xenopus and may be a general feature of NTS sequences.

Animals↗

PubMed1982

Rate of turnover of structural variants in the rDNA gene family of Drosophila melanogaster.

A high degree of polymorphism for the length and copy number of rDNA spacers, in both the X and Y chromosome clusters, has been found in a wild population of Drosophila melanogaster. The genetic behaviour of rDNA structural variants in separate and mixed populations derived from isofemale lines suggest that they are not subject to strong selection and are stable for over 1,000 generations. The high structural variability suggests an evolutionary rapid process of turnover in the family which could partly explain widespread sequence homogeneity (concerted evolution) of rDNA within a species.

Animals↗