Effect of mutations in a zinc-binding domain of yeast RNA polymerase C (III) on enzyme function and subunit association

The conserved amino-terminal region of the largest subunit of yeast RNA polymerase C is capable of binding zinc ions in vitro. By oligonucleotide-directed mutagenesis, we show that the putative zinc-binding motif CX2CX6-12CXGHXGX24-37CX2C, present in the largest subunit of all eukaryotic and archaebacterial RNA polymerases, is essential for the function of RNA polymerase C. All mutations in the invariant cysteine and histidine residues conferred a lethal phenotype. We also obtained two conditional thermosensitive mutants affecting this region. One of these produced a form of RNA polymerase C which was thermosensitive and unstable in vitro. This instability was correlated with the loss of three of the subunits which are specific to RNA polymerase C: C82, C34, and C31.