Nuclear cap binding complex (CBC) is recruited cotranscriptionally and stimulates spliceosome assembly on nascent mRNAs; however, its possible functions in regulating transcription elongation or termination were not well understood. We show that, while CBC appears to be dispensable for normal rates and processivity of elongation by RNA polymerase II (Pol II), it plays a direct role in preventing polyadenylation at weak termination sites. Similarly to Npl3p, with which it interacts, CBC suppresses the weak terminator of the gal10-Delta56 mutant allele by impeding recruitment of termination factors Pcf11p and Rna15p (subunits of cleavage factor IA [CF IA]) and does so without influencing Npl3p occupancy at the termination site. Importantly, deletion of CBC subunits or NPL3 also increases termination at a naturally occurring weak poly(A) site in the RNA14 coding sequences. We also show that CBC is most likely recruited directly to the cap of nascent transcripts rather than interacting first with transcriptional activators or the phosphorylated C-terminal domain of Pol II. Thus, our findings illuminate the mechanism of CBC recruitment and extend its function in Saccharomyces cerevisiae beyond mRNA splicing and degradation of aberrant nuclear mRNAs to include regulation of CF IA recruitment at poly(A) selection sites.
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