Poster
# 16
Poster |
|
6th Internet World Congress for Biomedical Sciences |
Monica Acosta(1), Kiyohito Yoshida(2)
(1)(2)Hokkaido University - Sapporo. Japan
|
| ||
[Cell Biology & Cytology] |
[Genetics & Bioinformatics] |
In Drosophila, suppressor mutations have, in general, a spontaneous origin and some of them are associated with the insertion of a transposable element into the genes. One of these classes presents structural characteristic similar to the retroviral provirus of the vertebrates. Among these suppressor mutations, the Hairy-wing, su(Hw) locus has been intensively studied in D. melanogaster. Most of su(Hw) alleles are caused by the insertion of gypsy (14,15). This retrotransposon presents its own transcription initiation signal that interacts with the regulatory sequences of adjacent genes, affecting their expression and causing the mutant phenotype (16). The studies on Om mutations suggest that it mechanism of action could be similar to su(Hw). In 1988, Dr. Hinton proposed that the mechanism by which the tomelement cause the Om mutations in D. ananassae is by an over-expression of those genes where tom inserts. In concordance with this hypothesis, it has been shown that the tom element presents it own promoter for the expression of the Om genes when activated by tissue-specific signals (17).
The Om genes suppressor effect it is observed only when the suppressor Omgenes are expressed with other Om genes; the Om(1K) gene do not suppress other mutations that affect the eye structure, as Pu and Lo or sng9 (11). The lack of information about the Drosophila suppressor genes make the Om genes and specially the Om(1J) region an interesting material for the study of the molecular mechanisms of suppressor mutations.
The analysis of Om(1J)Su34 showed that the mRNA detected in this region do not correspond to the suppressor gene. Other Om genes also present transcription domains near the tom insertion site that do not correspond to the mutated gene. The observations of Yoshida et al. (5) and Juni et al. (6) about the Om(2D) and Om(1E) genes, show that in Om(2D) there are at least four independent coding regions near the tom insertion site. One of them is the precursor gene for the OATtom near a gene do not ever promote it expression (5). In the Om(1E)53 allele, tom insertion site it is located 15 kb upstream respect to the Om(1E) transcription initiation site, and even though there are two transcripts in between this two regions, their expression do not result in a mutant phenotype (6). In Om(1A), Om(2D) and Om(1D), the site of tom insertion reside 70 Kb downstream the transcription initiation site (2,4,5).
The Om(1J) transcripts are expressed in both wild type and mutant individuals, without neither quantitative nor qualitative differences in the pattern of expression in the imaginal discs. With this data we are able to affirm that the transcripts detected in the Om(1J) region are not responsible for the suppressor effect, however their participation in the suppressor effect can not be discarded yet. The transformation of Om(1D) with a construction of hsp70 promoter and the messenger sequence, would give evidence of the function of these transcripts.
The cytological analysis of Om(1J)Su34 individuals gave additional information about the transcription domain of this gene. The brake point observed in the 6C region of the X chromosome in the Om(1J)Su34R8 and Om(1J)Su34R7 revertants reaffirm the location of the Om(1J) mutated locus to this site. This indicate that Om(1J)Su locus extends it transcription domain into this region, and that the tom element promoter effect over Om(1J)Su34R has been suppressed, possibly, due to the chromosomal rearrangements involving this region, specially the transposition involving the 6C region. Then we suppose that in Om(1J)Su34R the tom promoter effect over Om(1J)Su it is interrupted due to the separation of the Om(1J) transcription unit from the tom element. With the obtained data, it is not possible to assign the transcription unit 5´ or 3´ from the tom insertion site.
Respect to the complete reversion of the Om(1J)Su34 mutation, it is possible to propose that the revertants present a modification of the suppressor effect in the specific stage of the Om(1D)9 gene expression. There is a considerable expression of Om(1D) in the imaginal disc of the wild type and in the developing photoreceptors located posteriori to the morphogenetic furrow, where tom is also expressed (2,18). These results suggested to us a model of the mechanism by which the tom element conduce to obtain the Om(1D) mutants: early over-expression of the Om(1D) protein would cause the death of the undifferentiated cells in the region anterior to the morphogenetic furrow, without affecting the differentiating photoreceptors. It has been observed that this cellular effect it is suppressed when the Om(1D) mutation is combined with the Om(1K)Su mutations (11).
In general, the Om phenotype seems to be caused by an excessive early transcription of the Om genes (3,7,4,5) under the control of the tom element promoter effect (17). The model proposed for the suppressor effect in D. ananassae it is illustrated in Fig 7 The suppressor genes of the Om system would be activated by the tom element insertion into the Om genes (11). The early expression of the Om genes and the suppressor Om gene at the same time, would let both genes interact as in a normal development, avoiding the accumulation of the Om transcripts and allowing the eye development to continue in a normal way. The regulation of the suppressor effect would be dependent on the amount of the suppressor transcripts, in a feedback mechanism. This affirmation it is based on the experiments done by Hinton, where the Om(1J)Su suppressor effect showed to be dose-dependent: Su/Su > Su/+ > +/+ , and that the presence of extra copies of the suppressor transcript (Su/Su/Su) did not modify it effect. However, the concomitance expression of the Om suppressor genes and the Om mutations, in most of the cases do not result in a suppression of the mutant phenotype. It is supposed that the regulatory factors would act in a complementary way regulating the suppressor gene product and the tom element in the suppressed gene (Yoshida, personal communication). Even though the Om(1J)Su identification is still pendant, it is possible to assume that it gene product would be the result of the over-expression induced by the tom element promoter effect.
The analysis of the Om(1J)SuR mutations it is important in the characterization of the suppressor mutations. The Southern blot assay showed almost no differences in the mutants Om(1J) region, the cytological analysis showed the presence of various chromosomal rearrangements, some of which were around the 6C region of the X chromosome, the site assigned to the Om(1J) locus (12).
The tom element transposition frequency has not been studied yet, however, Hinton experiments (11) indicate that the frequency of Om mutants origin it is not reduced in the presence of Om suppressors. This indicate that the intrinsic functions necessary for the tom element transposition are not subject of suppression, at least in the primary oocytes, where the Om mutants originate. Then it is reasonable to think that the mobilization of these sequences in the genome is the result of transposition events.
The Om genes are normally involved in the eye development pathway, however, it similitude with D. melanogaster has been suggested just for two of them, the Om(1D) and Om(1A) genes. We do not doubt that these genes play a role in the eye developmental pathway, but it way of action and the specificity of the tom element insertion still need to be clarified.
|
|
| ||
|
[Cell Biology & Cytology] |
[Genetics & Bioinformatics] |
