Abstract
Using DNA encoding the largest subunit of Drosophila melanogaster RNA polymerase II, we isolated the homologous hamster RPO21 gene. Nucleotide sequencing of both the hamster and D. melanogaster RPO21 DNAs confirmed that the RPO21 polypeptides of these two species, like the Saccharomyces cerevisiae RPO21 polypeptide, contain both an N-terminal region homologous to the Escherichia coli RNA polymerase subunit beta' and a unique polymerase II-specific C-terminal domain. This C-terminal domain, encoded by separate exons in the D. melanogaster and hamster genes, consists of a tandemly repeated heptapeptide sequence. By constructing a series of deletions in DNA encoding the 26 heptapeptide repeats normally present in the S. cerevisiae RPO21 polypeptide, we have established that a minimum of between 9 and 11 repeats is necessary for RPO21 function in yeast cells. Replacement of the yeast RPO21 heptapeptide repeats by the longer hamster repetitive domain resulted in viable yeast cells with no detectable mutant phenotype, while a similar replacement of the yeast repeats by the more divergent D. melanogaster repeats was a recessive lethal mutation. We suggest that this novel repetitive domain is essential for proper initiation of transcription by RNA polymerase II and that it may mediate the functions of TATA boxes, upstream activating sequences, and enhancers.