Potential DNA replication accessory factors from the yeast Saccharomyces 
cerevisiae have previously been identified by their ability to bind to DNA 
polymerase alpha protein affinity matrices (J. Miles and T. Formosa, Proc. Natl. 
Acad. Sci. USA 89:1276-1280, 1992). We have now used genetic methods to 
characterize the gene encoding one of these DNA polymerase alpha-binding 
proteins (POB1) to determine whether it plays a role in DNA replication in vivo. 
We find that yeast cells lacking POB1 are viable but display a constellation of 
phenotypes indicating defective DNA metabolism. Populations of cells lacking 
POB1 accumulate abnormally high numbers of enlarged large-budded cells with a 
single nucleus at the neck of the bud. The average DNA content in a population 
of cells lacking POB1 is shifted toward the G2 value. These two phenotypes 
indicate that while the bulk of DNA replication is completed without POB1, 
mitosis is delayed. Deleting POB1 also causes elevated levels of both chromosome 
loss and genetic recombination, enhances the temperature sensitivity of cells 
with mutant DNA polymerase alpha genes, causes increased sensitivity to UV 
radiation in cells lacking a functional RAD9 checkpoint gene, and causes an 
increased probability of death in cells carrying a mutation in the MEC1 
checkpoint gene. The sequence of the POB1 gene indicates that it is identical to 
the CTF4 (CHL15) gene identified previously in screens for mutations that 
diminish the fidelity of chromosome transmission. These phenotypes are 
consistent with defective DNA metabolism in cells lacking POB1 and strongly 
suggest that this DNA polymerase alpha-binding protein plays a role in 
accurately duplicating the genome in vivo.