Suppressor of sable [Su(s)] and Wdr82 down-regulate RNA from heat-shock-inducible repetitive elements by a mechanism that involves transcription termination.

Although RNA polymerase II (Pol II) productively transcribes very long genes in vivo, transcription through extragenic sequences often terminates in the promoter-proximal region and the nascent RNA is degraded. Mechanisms that induce early termination and RNA degradation are not well understood in multicellular organisms. Here, we present evidence that the suppressor of sable [su(s)] regulatory pathway of Drosophila melanogaster plays a role in this process. We previously showed that Su(s) promotes exosome-mediated degradation of transcripts from endogenous repeated elements at an Hsp70 locus (Hsp70-αß elements). In this report, we identify Wdr82 as a component of this process and show that it works with Su(s) to inhibit Pol II elongation through Hsp70-αß elements. Furthermore, we show that the unstable transcripts produced during this process are polyadenylated at heterogeneous sites that lack canonical polyadenylation signals. We define two distinct regions that mediate this regulation. These results indicate that the Su(s) pathway promotes RNA degradation and transcription termination through a novel mechanism.

Drosophila suppressor of sable protein [Su(s)] promotes degradation of aberrant and transposon-derived RNAs.

RNA-binding proteins act at various stages of gene expression to regulate and fine-tune patterns of mRNA accumulation. One protein in this class is Drosophila Su(s), a nuclear protein that has been previously shown to inhibit the accumulation of mutant transcripts by an unknown mechanism. Here, we have identified several additional RNAs that are downregulated by Su(s). These Su(s) targets include cryptic wild-type transcripts from the developmentally regulated Sgs4 and ng1 genes, noncoding RNAs derived from tandemly repeated alphabeta/alphagamma elements within an Hsp70 locus, and aberrant transcripts induced by Hsp70 promoter transgenes inserted at ectopic sites. We used the alphabeta RNAs to investigate the mechanism of Su(s) function and obtained evidence that these transcripts are degraded by the nuclear exosome and that Su(s) promotes this process. Furthermore, we showed that the RNA binding domains of Su(s) are important for this effect and mapped the sequences involved to a 267-nucleotide region of an alphabeta element. Taken together, these results suggest that Su(s) binds to certain nascent transcripts and stimulates their degradation by the nuclear exosome.

Suppressor of sable, a putative RNA-processing protein, functions at the level of transcription.

The Drosophila melanogaster su(s) gene product negatively regulates the expression of mutant alleles with transposon insertions in the 5'-transcribed region by an unknown mechanism. We have investigated here su(s) function through in vivo structure-function analysis, heterologous reporter gene assays, and in vivo transcriptional induction experiments. We have shown that mutations of two arginine-rich motifs (ARMs), an acidic region, or two CCCH zinc fingers affect the ability of Su(s) to downregulate the expression of an insertion mutant allele and to autoregulate genomic su(s) transgenes. Using yeast and HeLa cell assays, we found that, when tethered to the promoter region, the N- and C-terminal regions of Su(s) can repress reporter gene expression, and all three motifs, but most significantly the ARMs, contribute to the repression activity. Finally, we showed that, in vivo, Su(s) inhibits the transcriptional induction of a transgene with an insertion in the first exon but does not affect induction of a similar transgene with a consensus 5' splice site near the upstream boundary of the insertion. Together, these results reveal a link between Su(s), transcription, and pre-mRNA processing.