Replication factor C (RFC) and the proliferating cell nuclear antigen (PCNA) are two essential DNA polymerase accessory proteins that are required for numerous aspects of DNA metabolism including DNA replication, DNA repair, and telomere metabolism. PCNA is a homotrimeric ring-shaped sliding DNA clamp that can facilitate DNA replication by tethering DNA polymerase delta or DNA polymerase epsilon to the DNA template. RFC is the 5-subunit multiprotein complex that loads PCNA onto DNA at primer-template junctions in an ATP-dependent reaction. All five of the RFC subunits share a set of related sequences (RFC boxes) that include nucleotide-binding consensus sequences. We report here that a mutation in the gene encoding the large subunit of yeast RFC gives rise to DNA metabolism defects that can be observed in vivo and in vitro. The rfc1-1 substitution (D513N) lies within the widely conserved RFC box VIII consensus sequence and results in phenotypes including DNA replication defects, increased sensitivity to DNA damaging agents, and elongated telomeres. Mutant Rfc1-1 complexes exhibit in vitro DNA replication defects that are sensitive to ATP concentrations, and these defects can be suppressed by mutant PCNA proteins which contain substitutions that destabilize the homotrimeric sliding DNA clamp.