Role for the Ssu72 C-terminal domain phosphatase in RNA polymerase II transcription elongation.
The RNA polymerase II (RNAP II) transcription cycle is accompanied by changes in the phosphorylation status of the C-terminal domain (CTD), a reiterated heptapeptide sequence (Y(1)S(2)P(3)T(4)S(5)P(6)S(7)) present at the C terminus of the largest RNAP II subunit. One of the enzymes involved in this process is Ssu72, a CTD phosphatase with specificity for serine-5-P. Here we report that the ssu72-2-encoded Ssu72-R129A protein is catalytically impaired in vitro and that the ssu72-2 mutant accumulates the serine-5-P form of RNAP II in vivo. An in vitro transcription system derived from the ssu72-2 mutant exhibits impaired elongation efficiency. Mutations in RPB1 and RPB2, the genes encoding the two largest subunits of RNAP II, were identified as suppressors of ssu72-2. The rpb1-1001 suppressor encodes an R1281A replacement, whereas rpb2-1001 encodes an R983G replacement. This information led us to identify the previously defined rpb2-4 and rpb2-10 alleles, which encode catalytically slow forms of RNAP II, as additional suppressors of ssu72-2. Furthermore, deletion of SPT4, which encodes a subunit of the Spt4-Spt5 early elongation complex, also suppresses ssu72-2, whereas the spt5-242 allele is suppressed by rpb2-1001. These results define Ssu72 as a transcription elongation factor. We propose a model in which Ssu72 catalyzes serine-5-P dephosphorylation subsequent to addition of the 7-methylguanosine cap on pre-mRNA in a manner that facilitates the RNAP II transition into the elongation stage of the transcription cycle.
Pubmed ID: 17101794 RIS Download
Alanine | Alleles | Amino Acid Sequence | Arginine | Carrier Proteins | Chromosomal Proteins, Non-Histone | DNA, Fungal | Models, Genetic | Molecular Sequence Data | Nuclear Proteins | Phenotype | Phosphoprotein Phosphatases | Protein Binding | Protein Structure, Secondary | RNA Polymerase II | Saccharomyces cerevisiae | Saccharomyces cerevisiae Proteins | Sequence Analysis, DNA | Suppression, Genetic | TATA-Binding Protein Associated Factors | Transcription Factor TFIID | Transcription, Genetic | Transcriptional Elongation Factors | mRNA Cleavage and Polyadenylation Factors