The large noncoding hsrω-n transcripts are essential for thermotolerance and remobilization of hnRNPs, HP1 and RNA polymerase II during recovery from heat shock in Drosophila

The hs-GAL4 (t) -driven expression of the hsr omega-RNAi transgene or EP93D allele of the noncoding hsr omega resulted in global down- or upregulation, respectively, of the large hsr omega-n transcripts following heat shock. Subsequent to temperature shock, hsr omega-null or those expressing hsr omega-RNAi or the EP93D allele displayed delayed lethality of most embryos, first or third instar larvae. Three-day-old hsr omega-null flies mostly died immediately or within a day after heat shock. Heat-shock-induced RNAi or EP expression in flies caused only a marginal lethality but severely affected oogenesis. EP allele or hsr omega-RNAi expression after heat shock did not affect heat shock puffs and Hsp70 synthesis. Both down- and upregulation of hsr omega-n transcripts suppressed reappearance of the hsr omega-n transcript-dependent nucleoplasmic omega speckles during recovery from heat shock. Hrp36, heterochromatin protein 1, and active RNA pol II in unstressed or heat-shocked wild-type or hsr omega-null larvae or those expressing the hs-GAL4 (t) -driven hsr omega-RNAi or the EP93D allele were comparably distributed on polytene chromosomes. Redistribution of these proteins to pre-stress locations after a 1- or 2-h recovery was severely compromised in glands with down- or upregulated levels of hsr omega-n transcripts after heat shock. The hsr omega-null unstressed cells always lacked omega speckles and little Hrp36 moved to any chromosome region following heat shock, and its relocation to chromosome regions during recovery was also incomplete. This present study reveals for the first time that the spatial restoration of key regulatory factors like hnRNPs, HP1, or RNA pol II to their pre-stress nuclear targets in cells recovering from thermal stress is dependent upon critical level of the large hsr omega-n noncoding RNA. In the absence of their relocation to pre-stress chromosome sites, normal developmental gene activity fails to be restored, which finally results in delayed organismal death.