RNA polymerase II of Schizosaccharomyces pombe contains twelve different subunits:: Identification and characterization of subunit Rpb4

In a comprehensive cDNA library of the fission yeast Schizosaccharomyces pombe, we have detected a cDNA which, when expressed in S. cerevisiae, is capable of the complete compensation of defects of the null allele rpb4 Delta 1::HIS3 of one of the specific subunit of RNA polymerase II of the budding yeast. Thus, using a functional approach, the gene was identified in the Sz. pombe genome encoding apparently the last, not yet characterized, component of the Sz. pombe RNA polymerase II, namely, subunit Rpb4, whose presence in the transcription apparatus of the fission yeast had been disputed (Gene, 1996. vol. 180, pp. 63-67; Gene, 1997, vol. 196, pp. 165-174). This gene contains three introns and is located on chromosome II. Comparison of the primary structure of subunit Rpb4 of Sz. pombe (135 aa; M 15 362 Da; pI 4.84), deduced from the cDNA, with the amino acid sequences of the orthologous subunits from Saccharomyces cerevisiae and other eukaryotes showed that the subunit Rpb4 of Sz. pombe is closer to the homologous subunits of higher eukaryotes, which do not contain the extended hydrophilic N-terminal domain present in subunit RPB4 of S. cerevisiae. The main structuring motif of subunit Rpb4 of the eukaryotic nuclear RNA polymerase II is probably the leucine zipper.