The 93D heat shock locus in Drosophila: a review

Abstract

One of the major heat shock induced genes of Drosophila melanogaster is located in the 93D6-7 bands of polytene chromosomes. The 93D locus shows many unique and intriguing features which are reviewed here. This locus is induced selectively and independently of the other heat shock loci by very diverse agents. The heat shock inducibility of this locus shows a remarkable dependence on the developmental history of the organism, its genetic background and other specific inducers of 93D. In all species ofDrosophila so far tested, one of the major heat shock loci is also selectively induced by the same set of 93D inducers and thus this locus is conserved in the genus. However, cloning and molecular characterisation of the primary DNA base sequence of the 93D ofD. melanogaster and the 93D-like locus ofD. hydei (the 2-48C locus) reveal that except for certain small stretches, the primary DNA base sequence of this locus has diverged rapidly. Interestingly, the structural organisation of the 93D ofD. melanogaster and 2-48C ofD. hydei is remarkably similar-both have a 5' unique sequence followed on the 3' end by a long stretch of a species-specific repetitive sequence apparently unique to the locus. Heat shock in both cases induces 3 major transcripts: a long 10-12 kb RNA covering the 5' unique and 3' repeat part, a 1.9-2.0 kb RNA complementary to the 5' unique part and a 1.2 kb transcript which lacks the 0.7 kb "intron" of the 2 kb RNA. None of the transcripts in either species has any appreciable open reading frames, and other evidence also suggests that the 93D or 2-48C loci do not code for any polypeptide. Functions of 93D or 93D-like loci remain uncertain although the locus is essential for ecdysone-induced differentiation of imaginal disks and viability of flies. Strong evidence exists that the activity pattern of the 93D heat shock locus inD. melanogaster distinctly influences heat shock puffing of the 87C locus, one of the two duplicate loci carrying the coding sequences for the major 70 kd heat shock polypeptide; presently available evidence suggests that this effect on 87C puffing is mediated via the mid-repetitive and heat-inducibleaß sequences residing at the 87C locus. The strong conservation of its inducible properties, of the organisation rather than the primary DNA base sequence and of the pattern of heat shock transcripts of 93D locus in the genusDrosophila point to important but hitherto little understood functions of this locus. The structure of its transcripts may have vital regulatory roles.