Genomic organization and functional analysis of a deletion variant of the 87A7 heat shock locus of Drosophila melanogaster

The major 70,000 M_r heat-induced protein (hsp 70) of Drosophila melanogaster is encoded at two loci, 87A7 and 87C1. The 87A7 locus has two hsp 70-coding sequences (z_c) about 2·8 × 10³ bases apart in a divergent orientation. We have identified a variant 87A7 locus, the Sze-13 chromosome, which has a reduced heat shock puff. Characterization of this altered 87A7 locus reveals that it has a small deletion in the spacer between the two genes that inactivates the proximal gene. The mutant locus has been cloned and the breakpoints of the deletion determined by sequence analysis. We find that the deletion removes the entire 5′ half of the proximal hsp 70 gene and extends to position -479 base-pairs upstream from the start of transcription of the distal gene. Using flies that retain only the mutant 87A7 hsp 70 locus of Sze-13 we have shown that the one remaining "intact" hsp 70 gene is still active. Since a partially deleted hsp 70 gene lacking the 3′ half of the transcription unit is also active, it is likely that the initial portion of the transcription unit (z_c) plus the conserved z_nc sequence that is immediately adjacent to the 5′ end of the gene contain the sequences necessary for hsp 70 expression.