REGULATION OF HEAT-SHOCK GENE INDUCTION AND EXPRESSION DURING DROSOPHILA DEVELOPMENT

Some heat shock genes are expressed in the absence of stress during em bryogenesis and metamorphosis in the fruit fly Drosophila melanogaster . Their functions in these processes are unknown. During development, each of the four members of the small heat shock protein family (Hsp27 , Hsp26, Hsp23 and Hsp22), which are coordinately induced in response to a heat stress, shows a specific pattern of expression in diverse ti ssues and cells. This expression is driven through cell-specific enhan cers in the promoter regions of their genes. In addition, some of the Hsps show cell-specific induction by heat shock. Hsp23, for example, i s only inducible in a single cell type (cone cells) of the eye ommatid ium, while the other small Hsps are inducible in all cells of the eve unit. In germ line tissues such as testes. Hsp23 and 27 are both readi ly expressed in the absence of stress (albeit in distinct cell lineage s) and cannot be further induced by heat shock. Hsp27 is expressed thr oughout oogenesis, but its intracellular localization is stage-specifi c, being nuclear from germarium to stage 6 and cytoplasmic from stage 8 onwards. Finally the small Hsps show tissue-specific post-translatio nal modifications. Thus the functions(s) of the small Hsps may be modu lated by different cell and developmental stage-specific mechanisms op erating either on their expression, their cellular localization or the ir structure by post-translational modifications.