Sequence evolution of the Drosophila heat shock locus hsr omega. I. The nonrepeated portion of the gene.

The locus which we now call hsr omega was originally identified as a large heat shock puff in polytene region 93D of Drosophila melanogaster. This puff was subsequently found to have several phenotypic characteristics that distinguished it from other heat shock puffs. These characteristics include induction by a number of agents that do not induce other puffs and the presence of large ribonucleotide particles that are not found elsewhere. Each Drosophila species has one heat shock puff with these phenotypes. In contrast to the strong sequence conservation seen in puffs coding for heat shock proteins, very little cross-hybridization is detected between hsr omega loci in different species, suggesting that the hsr omega loci are diverging rapidly. Comparative analyses of the hsr omega locus from D. melanogaster, D. pseudoobscura, and D. hydei show that, despite the sequence change, the structure of the locus and its transcripts has been conserved, along with a number of short regions of the sequence. The short regions of conservation offer some clues to the function of this unusual locus. In addition, these comparisons offer a view of the evolution of a gene whose primary function does not appear to be protein coding.