Hsp28stl: a P-element insertion mutation that alters the expression of a heat shock gene in Drosophila melanogaster.

We have identified and cloned a mutant allele of the small heat shock gene Hsp28 of Drosophila melanogaster. This allele, which we have called Hsp28stl, produces small amounts of a single aberrantly large, heat-inducible transcript in heat-shocked flies, while a normal-sized Hsp28 transcript is present only in fertile females. No Hsp28 transcript at all is detected in mutant prepupae, a stage when wildtype flies show high levels of Hsp28 RNA. We have cloned the Hsp28stl allele, and have found that a 1.3-kb defective P-element is present 5' to Hsp28 in the mutant line. The site of P-element insertion lies between the Hsp28 "TATA box" sequence and the Hsp28 RNA cap site; in contrast to previously described P-element insertions, the element at Hsp28stl is flanked by a two base pair duplication of the insertional target sequence. The results suggest that this insert may separate elements regulating heat-inducible and developmental expression of Hsp28, leading to the different patterns of transcription observed.