The 93D heat shock locus of Drosophila melanogaster: modulation by genetic and developmental factors

Abstract

The 93D locus in Drosophila melanogaster and the 93D-like loci in other species of Drosophila. collectively termed hsr ω (heat shock RNA omega) locus, display several unique and intriguing features: (i) developmental regulation and selective induction by several agents like benzamide, colchicine, thiamphenicol, vit-B₆: (ii) functional conservation in the genus but a very rapid DNA base sequence divergence: (iii) in spite of the rapid DNA sequence divergence, a strong conservation of organization (a 5' unique region and a 3' long tandem repeat region) and the pattern of multiple ω transcripts in the genus: (il') a general nontranslatability of all the three major species of ω transcripts (an ~IO-kb ωI, a 2.0-kb ω2, and a 1.2-kb ω3 species) although some recent evidence favours translatability of a small open reading frame (~23 -27 amino acid long) in Ihe ω3 Iranscript: (ν) dispensability of the hsr ω locus for heat shoek protein synthesis but indispensability for viability of flies. The heat siwek inducibility of the 93D locus of D. melanogaster is selectively repressed by (i) combination of heat shock with another inducer of 93S; (ii) rearing of larvae at I0ϐC: (iii) heterozygous deficiency for the 93D region: and (iv) conditions that alter levels of beta-alanine. In all cases of repression of the 93D locus during heat shock, the 87A and 87C loci (the two duplicate loci harbouring multiple copies for hsp70 and the alpha-beta repeat sequences (at 87C)) develop unequal puffs. The hsr ω locus appears to be under a complex system of regulation involving autoregulation as well as regulation by other factors in the cell which possibly operate through different control elements on the locus.