THE NUCLEUS-LIMITED HSR-OMEGA-N TRANSCRIPT IS A POLYADENYLATED RNA WITH A REGULATED INTRANUCLEAR TURNOVER

The Drosophila Hsr-omega puff, one of the largest heat shock puffs, re veals a very unusual gene, identified by heat shock but constitutively active in nearly all cell types. Surprisingly, Hsr-omega yields two t ranscription end-products with very different roles. The larger, omega -n, is a nuclear RNA with characteristics suggesting a new class of nu clear RNAs. Although it neither leaves the nucleus nor undergoes proce ssing, omega-n RNA is polyadenylated, showing that polyadenylation is not limited to cytoplasmic RNA, but possibly has a function in the nuc leus. The amount of omega-n within the nucleus is specifically regulat ed by both transcription and turnover. Heat shock and several other ag ents cause rapid increases in omega-n. A rapid return to constitutive levels follows withdrawal of the agents. Degradation of omega-n is inh ibited by actinomycin D, suggesting a novel intranuclear mechanism for RNA turnover. Within the nucleus, some omega-n RNA is concentrated at the transcription site; however, most is evenly distributed over the nucleus, showing no evidence of a concentration gradient which might b e produced by simple diffusion from the site of transcription. Previou s studies suggested that omega-n has a novel regulatory role in the nu cleus. The actinomycin D-sensitive degradation system makes possible r apid changes in the amount of omega-n, allowing the putative regulator y activities to reflect cellular conditions at a given time. Omega-n d iffers from the best studied nuclear RNAs, snRNAs, in many ways. Omega -n demonstrates the existence of intranuclear mechanisms for RNA turno ver and localization that may be used by a new class of nuclear RNAs.