Restrictor slows RNAPII elongation to promote termination at noncoding RNA loci.

The eukaryotic genome is broadly transcribed by RNA polymerase II (RNAPII) to produce protein-coding messenger RNAs (mRNAs) and a repertoire of noncoding RNAs (ncRNAs). Although RNAPII is very processive during mRNA transcription, it terminates rapidly during synthesis of many ncRNAs, particularly those that arise opportunistically from accessible chromatin at gene promoters or enhancers. The divergent fates of mRNA versus ncRNA species raise many questions about how RNAPII and associated machineries discriminate functional from spurious transcription. Restrictor, comprised of the RNA binding protein ZC3H4 and RNAPII-interacting protein WDR82, has been implicated in restraining the expression of ncRNAs. However, the determinants of Restrictor specificity and the mechanism of transcription suppression remain unclear. Here, we investigate Restrictor using unbiased sequence screens and rapid protein degradation followed by nascent RNA sequencing. We found that Restrictor promiscuously suppresses early elongation by RNAPII, but this activity is blocked at most mRNAs by the presence of a 5' splice site. Consequently, Restrictor is a critical determinant of transcription directionality at divergent promoters and prevents transcriptional interference. Mechanistically, we show that rather than terminating RNAPII directly, Restrictor acts by reducing the rate of transcription elongation, rendering RNAPII susceptible to early termination by other machineries.