| Summary: | The E(var)45-19 mutation of Drosophila melanogaster was isolated in a genetic
screen for P-element induced enhancers of the variegating rearrangement, wm4.
Remobilization of the P-element in E(var)45-19 resulted in a loss of its ability to enhance
position-effect variegation (PEV) of wm4, indicating that the P-element in this mutant
resulted in the E(var) phenotype. An allele of E(var)45-19, Su(var)r27, was isolated
following mobilization of the P-element. Su(var)r27 was demonstrated to have sex-specific
effects on the variegating rearrangements wm4 and bwVDe2. In addition to its effect on
PEV, Su(var)r27 was shown to enhance the mutant phenotype associated with the
retroelement-induced allele wbl, suggesting that this locus is involved in the regulation of
both chromatin structure and retroelement expression.
The P-element insert in E(var)45-19 was located in cytogenetic region 63A by in
situ hybridization. The P-element was shown to be inserted into the 3'LTR of a novel
retrovirus-like transposon, which I named nomad. DNA sequence analysis showed that
nomad contained three long ORFs that were similar to the gag, pol and env genes of
retroviruses and the copia-like elements of Drososphila melanogaster. The nomad element
terminates with 519 base pair long terminal repeats, each of which contains eukaryotic
consensus transcription initiation and termination signals, nomad elements are located at
approximately 10-15 sites within the euchromatic arms of the genome and at the
chromocenter as shown by in situ hybridization. The host DNA sequence TANA was
duplicated on each side of the nomad element and appears to be a prefered target site for
insertion of nomad elements. Analysis of the zinc finger motif in the pol gene product of
retrotransposons known to have target site preference suggests involvement of the integrase
subunit in target site selection for retrotransposons that display insert site specificity. A
comparison of the predicted amino acid sequence of the pol-like genes of several known
retrotransposons was made and the phylogenetic relationship between nomad and other
retrovirus-like mobile elements was determined. It was clear from this conceptual protein
analysis and from analysis of structural characteristics that retrotransposons of the gypsy
class can be generally classified as members of one of two distinct groups. The
phylogenetic relationships of these groups are also discussed.
The level of nomad transcription in the E(var)45-19 and Su(var)r27 mutations was
shown to correlate with their effect on PEV, suggesting that the nomad element may be
directly involved in the regulation of chromatin structure. In conclusion, a number of
speculative models are presented to explain the effect of mutations in the nomad element on
PEV and retroelement expression. [Scientific formulae used in this abstract could not be reproduced.] === Science, Faculty of === Zoology, Department of === Graduate
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