Biologjia Online

Ess1 is an essential peptidyl-prolyl cis/trans isomerase in the yeast Saccharomyces cerevisiae. Ess1 and its human homolog, Pin1, bind to phospho-Ser-Pro sites within proteins, including the carboxy-terminal domain (CTD) of Rpb1, the largest subunit of RNA polymerase II (pol II). Ess1 and Pin1 are thought to control mRNA synthesis by catalyzing conformational changes in Rpb1 that affect interaction of co-factors with the pol II transcription complex. Here we characterized wild-type and mutant Ess1 proteins in vitro and in vivo. We found that Ess1 preferentially binds and isomerizes CTD heptad-repeat (YSPYSPS) peptides that are phosphorylated on Ser5. Binding by the mutant proteins in vitro was essentially normal, and the proteins were mostly stable in vivo, however, their catalytic activity was reduced >1,000-fold. These data along with results of in vivo titration experiments indicate that Ess1 isomerase activity is required for growth, but only at vanishingly low levels. We found that although wild-type cells contain about ~200,000 molecules of Ess1, less than 400 molecules per cell are sufficient for growth. In contrast, higher levels of Ess1 were required for growth in the presence of certain metabolic inhibitors, suggesting that Ess1 is important for tolerance to environmental challenge.