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The complementary DNAs of the 12 subunits of fission yeast (Schizosaccharomyces pombe) RNA polymerase II were expressed from strong promoters in Saccharomyces cerevisiae and tested for heterospecific complementation by monitoring their ability to replace in vivo the null mutants of the corresponding host genes. Rpb1 and Rpb2, the two largest subunits and Rpb8, a small subunit shared by all three polymerases, failed to support growth in S. cerevisiae. The remaining nine subunits were all proficient for heterospecific complementation and led in most cases to a wild-type level of growth. The two α-like subunits (Rpb3 and Rpb11), however, did not support growth at high (37 °C) or low (25 °C) temperatures. In the case of Rpb3, growth was restored by increasing the gene dosage of the host Rpb11 or Rpb10 subunits, confirming previous evidence of a close genetic interaction between these three subunits.

ABC10α is a small polypeptide shared by the three yeast RNA polymerases. Homologous polypeptides in higher eukaryotes have a zinc-binding CX2C … CX2C motif and a conserved basic C-terminal end. These features are also found in archaeal gene products that may encode an RNA polymerase subunit. The CX2C … CX2C motif is partly dispensable, since only its first cysteine is essential for growth, whereas the basic C-terminal end is critical in vivo. A mutant in the latter domain has an RNA polymerase III-specific defect and, in vitro, impairs RNA polymerase III assembly. Polymerase activity was, however, not affected in various faithful transcription assays. The mutant is suppressed by a high gene dosage of the second largest subunit of RNA polymerase III, whereas the homologous subunits of RNA polymerase I and II have aggravating effects. In a two-hybrid assay, ABC10α binds to the C-terminal half of the second largest subunit of RNA polymerase I, in a way that requires the integrity of the CX2C … CX2C motif. Thus, ABC10α appears to interact directly with the second largest subunit during polymerase assembly. This interaction is presumably a major rate-limiting step in assembly, since diploid cells containing only one functional gene copy for ABC10α have a partial growth defect.