CTD-mediated recruitment of Yra1. Yra1 (possibly in complex with THO and Sub2) does not bind to Ser5P containing CTD and is not observed at high level at the 5′ end of genes. Yra1 is recruited to active genes during elongation when the CTD is doubly phosphorylated on Ser2 and Ser5. As it is not known whether Yra1 can bind both the CTD and TREX, it is modeled in both contexts. Yra1 remains bound to the CTD until the nascent mRNA reaches a certain length or reaches the CTD-bound Yra1; this would avert premature export attempts. At the 3′ end of the gene, Yra1 dissociates from the CTD, either because the phosphorylation pattern changes to predominant Ser2 phosphorylation (in which Yra1 does not bind) or because Yra1 binds the nascent mRNA with higher affinity.
