Remarkable Site Specificity of Local Transposition into the hsp70 Promoter of Drosophila melanogaster

Heat-shock genes have numerous features that ought to predispose them to insertional mutagenesis via transposition. To elucidate the evolvability of heat-shock genes via transposition, we have exploited local transposition and Drosophila melanogaster strains with EPgy2 insertions nearby the hsp70 gene cluster at 87A7 to produce numerous novel EPgy2 insertions into these hsp70 genes. More than 50% of 45 independent insertions were into two adjacent nucleotides in the proximal promoter at positions -96 and -97, and no insertions were into coding or 3'-flanking sequence. All inserted transposons were in inverse orientation to the starting transposon. The frequent insertion into nucleotides -96 and -97 is consistent with the DNase hypersensitivity, absence of nucleosomes, flanking GAGA factor binding sites, and nucleotide sequence of this region. These experimental insertions recapitulated many of the phenotypes of natural transposition into hsp70: reduced mRNA expression, less Hsp70 protein, and decreased inducible thermotolerance. The results suggest that the distinctive features of heat-shock promoters, which underlie the massive and rapid expression of heat-shock genes upon heat shock, also are a source of evolutionary variation on which natural selection can act.

Key Words: Drosophila, P element, hsp70, promoter, transposition

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