DNA sequences flanking the starts of the hsp 70 and αβ heat shock genes are homologous

The 87A7 and 87C1 cytogenetic loci of D. melanogaster contain two different types of heat-inducible genes. One of these codes for the 70,000-dalton heat shock protein (hsp 70) and is present at both loci, while the other gene, αβ, codes for an RNA of unknown function and is found only at 87C1. We find that restriction fragments that map adjacent to the 5′ end of the hsp 70 gene hybridize to fragments that map adjacent to the 5′ end of αβ. Electron microscopic mapping of duplex DNA formed between segments that contain these different genes reveals an extensive homologous region of several hundred base pairs that is adjacent to the start of the respective transcription units. A subcloned DNA segment that maps entirely within this homologous region hybridizes in situ exclusively to loci 87A and 87C on polytene chromosomes. We note in particular the absence of detectable hybridization at other heat shock loci-even with long autoradiographic exposure. We propose that the homology which is restricted to this subset of the heat shock genes is the result of a recent transposition of αβ DNA adjacent to a copy of the regulatory region of the hsp 70 gene, thereby placing expression of αβ under control of the heat shock response.