The non-coding transcripts of hsr-omega gene in Drosophila: Do they regulate trafficking and availability of nuclear RNA-processing factors?

The 93D or hsr-omega (hsrω) is an unusual non-protein-coding gene with multiple transcription products which are dynamically expressed in most cell types of Drosophila melanogaster and this gene, besides being a member of the heat shock gene family, is uniquely induced in polytene cells by a variety of amides. The various aspects of this gene's organization, regulation and inducible properties are briefly reviewed. Recent data in our laboratory show that absence of the hsr-omega transcripts because of nullosomy or over-expression of the these transcripts in specific cell types due to mutation in the promoter region of this gene results in specific phenotypes. It is known from several earlier and our recent studies that in unstressed cell nuclei a variety of hnRNA binding proteins (hnRNPs) associate with many chromosomal sites, including the 93D, and with extra-chromosomal speckles where the hsr-omega transcripts also co-localize. Following heat shock and other stresses, the bulk of these proteins and the hsr-omega nuclear (hsrω-n) transcripts get localized to the 93D site. We propose that one of the important functions of the hsrω-n transcripts is to act as a 'sink' for at least some members of the hnRNPs and related proteins so that any increase or decrease in the abundance of these nuclear transcripts correspondingly modifies the 'sink' size, which in turn affects the availability of such proteins in active nuclear compartments and regulates the nuclear RNA processing activity. It appears that non-availability or over-abundant availability of these transcripts disrupts the regulated and fine-tuned balance of the various RNA-processing factors resulting in trans-dominant mutant phenotypes. We believe that binding with specific proteins and consequent regulation of their activity may be a common feature of the functions of non-protein coding genes.