Heat shock response-regulation and functions of coding and non-coding heat shock genes in Drosophila

Heat shock response is a homeostatic adaptive response exhibited by nearly all cells in response to thermal and a variety of other stresses. In this article, the nature, functions and regulation of the various families of heat shock proteins are presented first and this is followed by an account of studies in the author's laboratory relating to the expression and regulation of some of the heat shock protein-coding genes and the non-coding hsr(tJgene in Drosophila. Our studies showed that unlike the common belief, different cell types of Drosophila exhibit differential activation of the various heat shock genes in a developmental stage and cell type specific manner. Transcripts of the different members of the HSP70 gene family are differentially induced and turned over in various cell types. A unique set of proteins, which includes the HSP64, is induced immediately after heat shock in larval Malpighian tubules while none of the typical heat shock proteins are induced in this tissue till at least one hour after heat shock. Interestingly, in most other tissues, HSP64 is not induced by heat shock. The heat inducible 93D or the hsrω gene of Drosophila melanogaster does not code for any protein. This gene is developmentally active and is also singularly induced by a variety of amides. Recent studies in our laboratory showed that one of its multiple non-coding transcripts is instrumental in the organization of nuclear omega speckles, which seem to be very important for regulation of availability of the heterogeneous nuclear RNA binding proteins (hnRNPs) for RNA processing activities in unstressed and stressed conditions as per contemporary requirements of the cell. Accordingly, over- as well as non-expression of this gene under normal conditions has severe consequences. It is likely that an equivalent of the non-coding hsrω gene is present in all other organisms as well. Future studies are expected to reveal the physiological bases of such pronounced as well as subtle differences in the stress response mounted by different cells of an organism and their evolutionary significance