Characterizing the Roles of Pif1 Family Helicase and N-terminus Domains in vivo

Abstract

Pif1 family helicases are 5’->3’ helicases found in nearly all eukaryotes and many prokaryotes that display high (>30%) levels of homology in their helicase domains. In Saccharomyces cerevisiae, there are two Pif1 family members, ScPif1 and ScRrm3 that perform diverse functions despite their remarkable similarity. ScPif1 inhibits telomerase, generates long 5’ flaps during Okazaki fragment maturation, unwinds G-quadruplex DNA secondary structures, and inhibits replication fork progression at tracts of rDNA, among other activities. ScRrm3’s primary function, meanwhile, is the disruption of nonnucleosomal protein-DNA complexes to promote replication fork progression through various sites in the genome, such as telomeres, rDNA, and tRNA genes. Because their helicase domains are highly similar, it is hypothesized that some of the ScPif1 vs. ScRrm3 functional specificity is derived from their divergent amino (N-) and carboxyl (C-) termini. This study used various assays to examine ScPif1, ScRrm3, and the bacterial homologs BacPif1 and CamPif1, along with N-terminal truncation and chimeric fusion proteins, to determine what role(s) the helicase and N-terminal domains play in vivo. Initial findings suggest that the helicase domain is largely responsible for ScPif1 activity at telomeres and Okazaki fragments, with a smaller role for the N-terminus. However, the ScRrm3 N-terminus appears to be both important for replication fork progression through protein-DNA complexes and capable of conferring this activity when fused to various Pif1 family homolog helicase domains.