Cloning of the esterase-5 locus from Drosophila pseudoobscura and comparison with its homologue in D. melanogaster

Abstract

A clone of the esterase-5 (Est-5) gene from Drosophila pseudoobscura has been isolated by hybridization to the cloned Est-6 gene of D. melanogaster. Southern analysis and sequencing of the cloned DNA revealed three regions of similarity to Est-6 that have been tentatively identified as genes, Est-5A, Est-5B, and Est-5C. Introduction of each of the three genes separately into D. melanogaster by P-element transformation has demonstrated that Est-5B encodes an enzyme with the same physical properties as EST 5 in D. pseudoobscura. Sequence analysis indicates that Est-5B encodes a 545-amino-acid protein and is composed of two exons separated by a 55-bp intron in the same position as the 51-bp intron in Est-6. Comparison of the Est-5B coding region with that of Est-6 reveals an overall similarity (73% at both the nucleotide and amino acid levels) that is substantially lower than that for other genes sequenced in both of these species. Total nucleotide and nonsynonymous site differences between Est-6 and Est-5B are more abundant in the second exon than in the first, suggesting differential effects of selection or mutation on these two exons. Comparisons of the 5'-flanking DNA of Est-5B and Est-6 reveal four short conserved sequence elements, but the remaining upstream sequences show no significant similarity. Conservation in the 3'-flanking DNA is limited to the presence of two polyadenylation sites that may correlate with the existence of two transcripts from both Est-5B and Est-6. The patterns of nucleotide substitutions and amino acid replacements between Est-5B and Est-6 are consistent with the hypothesis that mutation and genetic drift are responsible for the differences between these two genes.