Suppressors of the temperature sensitivity of DNA polymerase α mutations in Saccharomyces cerevisiae

We have isolated two high copy, allele-specific suppressors of the temperature sensitivity of mutations in POLI, the gene that encodes the catalytic subunit of DNA polymerase alpha in the yeast Saccharomyces cerevisiae. Both genes, PSP1 and PSP2, also partially suppressed a mutation in POL3 which encodes DNA polymerase delta, and both also affected a mutation in CDC6, which acts in initiation of DNA replication. Suppression was not general, since ts mutations in several genes unrelated to replication were not affected. PSP1 was partially effective on low-copy-number vectors, while PSP2 required high copy numbers. The presence of suppressing plasmids did not alter the steady-state level of Poll protein, so suppression does not appear to be due to an increase in production or stability of Pol1p. Deletion of either PSP gene or both in combination resulted in apparently normal viable cells. While neither gene is homologous to genes with known functions, PSP1 and PSP2 both have unusual amino acid compositions: PSP1 is rich in asparagine and glutamine, while PSP2 is rich in asparagine and contains "RGG" motifs that have been associated with RNA-binding proteins. We also describe a transposon-mediated strategy that should be generally effective for rapid characterization of multicopy suppressors.