The multifunctional protein p54nrb/PSF recruits the exonuclease XRN2 to facilitate pre-mRNA 3′ processing and transcription termination

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Abstract
Termination of RNA polymerase II transcription frequently requires a poly(A) signal and cleavage/polyadenylation factors. Recent work has shown that degradation of the downstream cleaved RNA by the exonuclease XRN2 promotes termination, but how XRN2 functions with 3′-processing factors to elicit termination remains unclear. Here we show that XRN2 physically associates with 3′-processing factors and accumulates at the 3′ end of a transcribed gene. In vitro 3′-processing assays show that XRN2 is necessary to degrade the downstream RNA, but is not required for 3′ cleavage. Significantly, degradation of the 3′-cleaved RNA was stimulated when coupled to cleavage. Unexpectedly, while investigating how XRN2 is recruited to the 3′-processing machinery, we found that XRN2 associates with p54nrb/NonO(p54)–protein-associated splicing factor (PSF), multifunctional proteins involved in several nuclear processes. Strikingly, p54 is also required for degradation of the 3′-cleaved RNA in vitro. p54 is present along the length of genes, and small interfering RNA (siRNA)-mediated knockdown leads to defects in XRN2 recruitment and termination. Together, our data indicate that p54nrb/PSF functions in recruitment of XRN2 to facilitate pre-mRNA 3′ processing and transcription termination.