Sequence-specific RNA recognition drives Restrictor-mediated termination of extragenic transcription.

Mammalian genomes harbor hundreds of thousands of RNA polymerase II (RNA Pol II) landing pads, enhancers, and promoters from which transcription initiates bidirectionally. Nevertheless, processive transcription is largely restricted to the small gene-containing fraction of the genome. An essential metazoan complex, Restrictor, composed of WDR82 and ZC3H4, restrains processive RNA Pol II activity at extragenic transcription units, thus representing a critical enforcer of genome utilization. However, because of the widespread recruitment of Restrictor to both genic and non-genic transcription sites, the mechanistic basis for its selectivity for extragenic transcription is unclear. Here, we show that while WDR82 tethers Restrictor to transcription initiation sites, the C3H1-type zinc fingers of ZC3H4 make sequence-specific interactions with motifs enriched at the 5' end of extragenic transcripts, with such interactions being required for transcription termination. Hence, although Restrictor recruitment requires WDR82-dependent tethering to the initiating RNA Pol II, its selectivity mainly arises from sequence-specific RNA recognition.