A DNA segment isolated from chromosomal site 67B in D. melanogaster contains four closely linked heat-shock genes.

cDNA clones coding for two different small heat-shock polypeptides were isolated. Both clones hybridize exclusively to the heat-shock puff site 67B, and restriction mapping of embryonic Drosophila melanogaster DNA showed that the two genes probably occur as single copies and are closely linked. The analysis was extended by isolating genomic clones, which contain these genes and two additional ones. The four different genes code for heat-induced poly(A)+ RNAs. These genes are clustered within an 11 kb segment and are separated by spacers of 1.0-4.7 kb. Three of the genes were found to exhibit alternating polarities. Thus in spite of their close linkage, the four heat-induced genes are most likely organized in individual transcription units.