A genetic look at the active site of RNA polymerase III

rpc160-112 a mutant of the RNA polymerase III active site, is I corrected i n vivo by six second-site mutants obtained by random mutagenesis. These mut ants introduce single-site amino acid replacements at the two large subunit s of the enzyme. The mutated motifs are conserved in RNA polymerases I and II and, for some of them, in the bacterial enzyme, thus delineating key ele ments of the active site in eukaryotic RNA polymerases.