Evolution and expression of the Sgs-3 glue gene of Drosophila.

A cluster of three glue genes is present at chromosomal site 68C in the Drosophila melanogaster genome. In this study, we have used a comparative approach to investigate both the regulation and the evolution of the largest of these three genes, Sgs-3. The homologous genes from two related Drosophila species (D. erecta and D. yakuba) have been introduced into the D. melanogaster genome by P-factor-mediated transformation. When the resulting transformant lines were assayed for expression of the introduced genes, near-normal patterns of expression were seen. This demonstrates that the cis-acting regulatory sequences of the introduced Sgs-3-homologous glue genes are capable of interacting effectively with the transcriptional machinery of D. melanogaster. We have also determined the sequences of the Sgs-3-homologous glue genes from D. simulans, D. erecta and D. yakuba. These sequences were compared and used in two ways. The first was to locate conserved sequence elements in regions known to be involved in regulation of the gene. Several such elements were found; they represent potential sites of cis-acting regulatory sequences. Second, we looked at the evolution of the glue gene protein-coding regions. A very rapidly evolving central region of the protein-coding sequences was found; this region contains a striking series of tandem repeats of a five amino acid residue sequence in all four species. Also a number of conserved aspects of the Sgs-3-homologous proteins were found; these features may be essential to their function as a glue.