The rpb4 subunit of fission yeast Schizosaccharomyces pombe RNA polymeraseII is essential for cell viability and similar in structure to the corresponding subunits of higher eukaryotes

Both the gene and the cDNA encoding the Rpb4 subunit of RNA polymerase II w ere cloned from the fission yeast Schizosaccharomyees pombe, The cDNA seque nce indicates that Rpb4 consists of 135 amino acid residues with a molecula r weight of 15,362. As in the case of the corresponding subunits from highe r eukaryotes such as humans and the plant Arabidopsis thaliana, Rpb4 is sma ller than RPB4 from the budding yeast Saccharomyces cerevisiae and lacks se veral segments, which are present in the S, cerevisiae RPB4 subunit, includ ing the highly charged sequence in the central portion. The RPB4 subunit of S, cerevisiae is not essential for normal cell growth but is required for cell viability under stress conditions, In contrast, S, pombe Rpb4 was foun d to be essential even under normal growth conditions. The fraction of RNA polymerase II containing RPB4 in exponentially growing cells of S. cerevisi ae is about 20%, but S. pombe RNA polymerase II contains the stoichiometric amount of Rpb4 even at the exponential growth phase. In contrast to the RP B4 homologues from higher eukaryotes, however, S. pombe Rpb4 formed stable hybrid heterodimers with S. cerevisiae RPB7, suggesting that S. pombe Rpb4 is similar, in its structure and essential role in cell viability, to the c orresponding subunits from higher eukaryotes. However, S. pombe Rpb4 is clo ser in certain molecular functions to S. cerevisiae RPB4 than the eukaryoti c RPB4 homologues.