Mutants in ABC10β, a conserved subunit shared by all three yeast RNA polymerases, specifically affect RNA polymerase I assembly

ABC10 beta, a small polypeptide common to the three yeast RNA polymerases, has close homology to the N subunit of the archaeal enzyme and is remotely related to the smallest subunit of vaccinial RNA polymerase, The eucaryotic, archaeal, and viral polypeptides share an invariant motif CX,C.., CC that is strictly essential for yeast growth, as shown by site-directed mutagenesis, whereas the rest of the ABC10 beta sequence is fairly tolerant to amino acid replacements. ABC10 beta has Zn2+ binding properties in vitro, and the CX2C ... CC motif may therefore define an atypical metal-chelating site. Hybrid subunits that derive most of their amino acids from the archaeal subunit are functional in yeast, indicating that the archaeal and eucaryotic polypeptides have a largely equivalent role in the organization of their respective transcription complexes. However, all eucaryotic forms of ABC10 beta harbor a HVDLIEK motif that, when mutated or replaced by its archaeal counterpart, leads to a polymerase I-specific lethal defect in vivo. This is accompanied by a specific lack in the largest subunit of RNA polymerase I (A190) in cell-free extracts, showing that the mutant enzyme is not properly assembled in vivo.