Characterization of the rpa43+ cDNA of Schizosaccharomyces pombe:: Structural similarity of subunit Rpa43 of RNA polymerase I and subunit Rpc25 of RNA polymerase III

We isolated and characterized full-length cDNA of the rpa43(+) gene encoding one of subunits of the nuclear RNA polymerase I of Schizosaccharomyces pombe. The gene contains two introns and is located on chromosome II. Comparison of the primary structure of the subunit Rpa43 of Sz. pombe (173 aa; M 19 385 Da; pI 5.36), deduced from the cDNA obtained, with the amino acid sequences of subunits A43 from Saccharomyces cerevisiae and Drosophila melanogaster demonstrates a high divergence of this protein in evolution. A comparison of the Rpa43 with other proteins from the SwissProt database revealed a similarity of this subunit to subunit Rpc25 of RNA polymerase III, which, as was shown previously, is structurally similar to subunit Rpb7 of RNA polymerase II. Thus, including the Rpa43 <-> Rpc25 <-> Rpb7 family, nuclear RNA polymerases I-III contain at least Il identical and/or similar subunits. This fact illustrates a pronounced resemblance of the organization of all three enzymes of the eukaryotic transcription apparatus. Moreover, at least ten out of these eleven families of eukaryotic RNA polymerase subunits have homologues in the 13-subunit archaeal RNA polymerase.