FIG. 7.

A model for how Rad6/H2B ubiquitylation becomes associated with the GAL1 gene and regulates H3 K4 methylation. (A) Based on recent studies and those herein, the recruitment of the Gal4 activator to the UAS in the promoter of the GAL1 gene results in the corecruitment of chromatin remodeling activities and transcriptional coregulators, such as Rad6/Bre1, SAGA, and mediator. Although Rad6/Bre1, SAGA, and mediator are depicted together, we do not suggest that they physically interact and have not indicated their ordered recruitment to the GAL1 promoter. In addition, with the exception of the nucleosome-free region at the upstream activation sequence, we do not accurately depict the nucleosome density of this promoter. Upon recruitment of Pol II and its associated general transcription factors (GTFs), the TFIIH-associated kinase, Kin28, phosphorylates the serine 5 (S5) position in the CTD to initiate transcription elongation. This phosphorylation results in the recruitment of transcription elongation factors to Pol II, such as the Paf1 complex, which in turn corecruits the Set1 HMT. (B) During or after the transcription initiation event and recruitment of the Paf1 complex, Rad6/Bre1 becomes associated with Pol II via the Paf1 complex in a mechanism we describe as a hand-off event. This handoff, or newly formed association of Rad6/Bre1 with Pol II, appears to be essential for the stimulation of Rad6's H2B-modifying activity, which, we propose, leads to localized nucleosome disruption around Pol II that is important for elongation. These disrupted nucleosomes may be those associated with the first or earliest rounds of new transcription, as Rad6 and ubH2B appear across the gene at the same time as Pol II's first appearance but occur transiently. The nucleosome disruption proposed to occur with ubH2B may also include the actions of the 19S proteasomal ATPases, which are recruited to chromatin in a H2B ubiquitylation-dependent manner and are important for H3 methylation (15). Given that Set1 (and perhaps Dot1; see text) is also associated with the Paf1 complex, we suggest that the localized disruption of nucleosomes caused by H2B ubiquitylation allows Set1 to access the core nucleosomes and histone tails for methylation of H3 on lysine 4. However, unlike histone methylation, H2B ubiquitylation is transient, and thus this modification's removal reverses the nucleosomal disruption that we propose to track with Pol II during gene transcription. (C and D) As Pol II moves and clears the promoter, S5 phosphorylation on the CTD is removed and followed by the recruitment of Ctk1, which phosphorylates the serine 2 (S2) position in the CTD and results in the direct association of Set2 with Pol II. Notably, Set1 is removed from the elongating Pol II complex shortly after initiation and loss of S5 phosphorylation; however, the Paf1 complex with Rad6/Bre1 remains associated throughout the length of the gene (Fig. 4; see references 26 and 49). Collectively, our results show that Rad6 travels with Pol II to mediate H2B ubiquitylation, a modification that appears important for the elongation cycle of newly activated transcription and the precise establishment of H3 methylation patterns.