Analysis of the essential functions of the C-terminal protein/protein interaction domain of Saccharomyces cerevisiae pol epsilon and its unexpected ability to support growth in the absence of the DNA polymerase domain

As first observed by Wittenberg (Kesti, T., Flick, K., Keranen, S., Syvaoja , J. E., and Wittenburg, C. (1999) Mel. Cell 3, 679-685), we find that dele tion mutants lacking the entire N-terminal DNA polymerase domain of yeast p ol epsilon are viable. However, we now show that point mutations in DNA pol ymerase catalytic residues of pol epsilon are lethal. Taken together, the p henotypes of the deletion and the point mutants suggest that the polymerase of pol epsilon may normally participate in DNA replication but that anothe r polymerase can substitute in its complete absence. Substitution is ineffi cient because the deletion mutants have serious defects in DNA replication. This observation raises the question of what is the essential function of the C-terminal half of pol epsilon, We show that the ability of the C-termi nal half of the polymerase to support growth is disrupted by mutations in t he cysteine-rich region, which disrupts both dimerization of the POL2 gene product and interaction with the essential DPB2 subunit, suggesting that th is region plays an important architectural role at the replication fork eve n in the absence of the polymerase function. Finally, the S phase checkpoin t, with respect to both induction of RNR3 transcription and cell cycle arre st, is intact in cells where replication is supported only by the C-termina l half of pol epsilon, but it is disrupted in mutants affecting the cystein e-rich region, suggesting that this domain directly affects the checkpoint rather than acting through the N-terminal polymerase active site.