CLONING OF AN RNA-POLYMERASE SUBUNIT CDNA OF THE FISSION YEAST SCHIZOSACCHAROMYCES-POMBE BY HETEROSPECIFIC COMPLEMENTATION IN SACCHAROMYCES-CEREVISIAE

The rph10 cDNA of the fission yeast Schizosaccharomyces pombe, encodin g one of the five small subunits common to all three nuclear DNA-depen dent RNA polymerases, was isolated from an expression cDNA library by two independent approaches: PCR-based screening and direct suppression by means of heterospecific complementation of a temperature-sensitive mutant defective in the corresponding gene of Saccharomyces cerevisia e. The cloned St. pombe cDNA encodes a protein Rpb10 of 71 amino acids with an M of 8275 Da, sharing 51 amino acids (71% identity) with the subunit ABC10 beta of RNA polymerases I-III from S, cerevisiae. All eu karyotic members of this protein family have the same general organiza tion featuring two highly conserved motifs (RCFT/SCGK and RYCCRRM) aro und an atypical zinc finger and an additional invariant HVDLIEK motif toward the C-terminal end. The last motif is only characteristic for h omologs from eukaryotes. In keeping with this remarkable structural co nservation, the St. pombe cDNA also fully complemented a S. cerevisiae deletion mutant lacking subunit ABC10 beta (null allele rph10-Delta 1 ::HIS3).