CTF4 and CTF18 are required for high-fidelity chromosome segregation. Both 
exhibit genetic and physical ties to replication fork constituents. We find that 
absence of either CTF4 or CTF18 causes sister chromatid cohesion failure and 
leads to a preanaphase accumulation of cells that depends on the spindle 
assembly checkpoint. The physical and genetic interactions between CTF4, CTF18, 
and core components of replication fork complexes observed in this study and 
others suggest that both gene products act in association with the replication 
fork to facilitate sister chromatid cohesion. We find that Ctf18p, an RFC1-like 
protein, directly interacts with Rfc2p, Rfc3p, Rfc4p, and Rfc5p. However, Ctf18p 
is not a component of biochemically purified proliferating cell nuclear antigen 
loading RF-C, suggesting the presence of a discrete complex containing Ctf18p, 
Rfc2p, Rfc3p, Rfc4p, and Rfc5p. Recent identification and characterization of 
the budding yeast polymerase kappa, encoded by TRF4, strongly supports a 
hypothesis that the DNA replication machinery is required for proper sister 
chromatid cohesion. Analogous to the polymerase switching role of the bacterial 
and human RF-C complexes, we propose that budding yeast RF-C(CTF18) may be 
involved in a polymerase switch event that facilities sister chromatid cohesion. 
The requirement for CTF4 and CTF18 in robust cohesion identifies novel roles for 
replication accessory proteins in this process.