HUMAN RNA-POLYMERASE-II SUBUNIT HSRPB7 FUNCTIONS IN YEAST AND INFLUENCES STRESS SURVIVAL AND CELL MORPHOLOGY

Using a screen to identify human genes that promote pseudohyphal conve rsion in Saccharomyces cerevisiae, we obtained a cDNA encoding hsRPB7, a human homologue of the seventh largest subunit of yeast RNA polymer ase II (RPB7). Overexpression of yeast RPB7 in a comparable strain bac kground caused more pronounced cell elongation than overexpression of hsRPB7. hsRPB7 sequence and function are strongly conserved with its y east counterpart because its expression can rescue deletion of the ess ential RPB7 gene at moderate temperatures. Further, immuno-precipitati on of RNA polymerase II from yeast cells containing hsRPB7 revealed th at the hsRPB7 assembles the complete set of 11 other yeast subunits. H owever, at temperature extremes and during maintenance at stationary p hase, hsRPB7-containing yeast cells lose viability rapidly, stress-sen sitive phenotypes reminiscent of those associated with deletion of the RPB4 subunit with which RPB7 normally complexes. Two-hybrid analysis revealed that although hsRPB7 and RPB4 interact, the association is of lower affinity than the RPB4-RPB7 interaction, providing a probable m echanism for the failure of hsRPB7 to fully function in yeast cells at high and low temperatures. Finally, surprisingly, hsRPB7 RNA In human cells is expressed in a tissue-specific pattern that differs from tha t of the RNA polymerase II largest subunit, implying a potential regul atory role for hsRPB7. Taken together, these results suggest that some RPB7 functions may be analogous to those possessed by the stress-spec ific prokaryotic sigma factor rpoS.