Figure 8.

Schematic model showing the hypothetical localization of Sub1 during transcription initiation. Sub1 is bound to the promoter by interacting with upstream DNA at the junction between single- and double-stranded DNA (36) through its DNA Binding Domain (Sub1 DBD) (Figure 5). The proposed localization of Sub1 in this model explains the reported physical (solid arrow) and genetic (dashed arrow) interaction of Sub1 with TFIIB (35,50). The intrinsically-disordered C-terminal domain of Sub1 (Sub1 CT) may extend to directly interact with the Rpb4/7 heterodimer, as suggested by the specific genetic interaction between this domain and RPB4 (Figure 5), and the physical direct interaction with Rpb7 (Figure 4). The genetic interaction between Sub1 and Rpb1 clamp (Figure 2C) is also illustrated. In addition, the model shows the connections of Fcp1 phosphatase with Rpb4 and Sub1 revealed by our genetic (Allepuz-Fuster et al., Calvo & Manley, 2005) and CoIP assays (Figure 6), and consistent with the structural data (76). Our proposed model suggests that Sub1 may influence Rpb1-CTD phosphorylation (25) and Rpb1 clamp functionality through its interaction with Rpb4/7, which could also explain the already suggested role of Sub1 both during transcription initiation (36,37) and elongation (38).