The carboxy-terminal domain (CTD) code encrypted within the Y1S2P3T4S5P6S7heptad repeats of RNA polymerase II (Pol2) is deeply rooted in eukaryal biology. Key steps to deciphering the code are identifying the events in gene expression that are governed by individual “letters” and then defining a vocabulary of multiletter “words” and their meaning. Thr4 and Ser7 exert opposite effects on the fission yeastpho1gene, expression of which is repressed under phosphate-replete conditions by transcription of an upstream flanking long noncoding RNA (lncRNA). Here we attribute the derepression ofpho1by a CTD-S7Amutation to precocious termination of lncRNA synthesis, an effect that is erased by mutations of cleavage-polyadenylation factor (CPF) subunits Ctf1, Ssu72, Ppn1, Swd22, and Dis2 and termination factor Rhn1. By contrast, a CTD-T4Amutation hyperrepressespho1, as do CPF subunit and Rhn1 mutations, implying thatT4Areduces lncRNA termination. Moreover, CTD-T4Ais synthetically lethal withppn1∆ andswd22∆, signifying that Thr4 and the Ppn1•Swd22 module play important, functionally redundant roles in promoting Pol2 termination. We find that Ppn1 and Swd22 become essential for viability when the CTD array is curtailed and thatS7Aovercomes the need for Ppn1•Swd22 in the short CTD context. Mutational synergies highlight redundant essential functions of (i) Ppn1•Swd22 and Rhn1, (ii) Ppn1•Swd22 and Ctf1, and (iii) Ssu72 and Dis2 phosphatases. CTD allelesY1F,S2A, andT4Ahave overlapping synthetic lethalities withppn1∆ andswd22∆, suggesting that Tyr1-Ser2-Thr4 form a three-letter CTD word that abets termination, with Rhn1 being a likely “reader” of this word.
Molecular assembly of RNA polymerase II from the fission yeast Schizosaccharomyces pombe: subunit-subunit contact network involving Rpb5
