Figure 3.

Control of RiBi and RP gene transcription by TORC1. RiBi factors are required for the proper expression, processing, assembly, export, and maturation of rRNA and RPs into ribosomes. This energetically costly procedure is under tight regulation, particularly at the transcription level. TORC1 regulates RiBi and RP gene transcription via multiple pathways: (1) TORC1 directly phosphorylates the Split Zn-finger transcription factor Sfp1, which presumably regulates its nuclear localization and/or binding to RP and possibly RiBi gene promoters to stimulate their expression. (2) Fhl1 and Rap1 bind constitutively to RP promoters. When TORC1 is active, phosphorylated Ifh1 binds to Fhl1 to stimulate transcription, possibly by recruiting the NuA4 histone acetyltransferase. When TORC1 is inactive, Yak1 phosphorylates Crf1, which subsequently outcompetes Ifh1 for binding to Fhl1. (3) Sch9 phosphorylates and thus inhibits Stb3 and the paralogs Dot6 and Tod6. Inhibition of TORC1/Sch9 results in the dephosphorylation of these three transcription repressors, which subsequently bind to RRPE and PAC elements found in RiBi promoters. Stb3 additionally binds RP promoters. Bound to promoters, these repressors recruit the RPD3L histone deacetylase complex to repress transcription.