The role of Chl1 DNA helicase in cohesion deposition and establishment

Abstract Sister chromatid cohesion is crucial for the accurate transmission of genetic material during cell division. The conserved family of cohesin proteins that mediate sister chromatid tethering reactions require Scc2, Scc4 for chromatin association and Eco1 for conversion to a tethering competent state. The mechanism by which cohesin proteins mediate cohesion establishment between newly replicated sister chromatids remains elusive. Popular models posit that cohesins loaded in front of the replication fork is modified by Eco1 and conformational changes in the cohesin complex allows the replication fork to pass through cohesin barriers thereby converting cohesins into a cohesion competent state to enable capture of replicated sister chromatids. This study provides new evidence that challenges previous notions of cohesion establishment. I use genetic and biochemical studies that link Eco1 with the Okazaki fragment maturation endonuclease Fen1. Furthermore I show genetic and physical interactions between Fen1 and the DNA helicase Chl1, which was previously identified to interact with Eco1 and play a role in sister chromatid cohesion. A detailed investigation of the Chl1 DNA helicase and its role in sister chromatid cohesion revealed its role in regulating cohesin and Scc2 deposition specifically during the S phase. Taken together, my studies suggest a new model of cohesion establishment wherein cohesins loaded in the S phase is modified by Eco1 behind the replication fork and mediates cohesion establishment. Further analysis of Chl1 also revealed the role of Chl1 in the deposition of condensin proteins. My results identify a novel link between the molecular mechanisms of sister chromatid cohesion with DNA condensation and suggests that these cellular processes are linked temporally and mechanistically. Detailed analysis of the Chl1 helicase reveals important novel functions in DNA metabolism and facilitates a better understanding of its clinically important human homologues, hChlR1 and BACH1.