DNA Polymerase α (swi7) and the Flap Endonuclease Fen1 (rad2) Act Together in the S-Phase Alkylation Damage Response in S. pombe

Polymerase α is an essential enzyme mainly mediating Okazaki fragment synthesis during lagging strand replication. A specific point mutation in Schizosaccharomyces pombe polymerase α named swi7-1, abolishes imprinting required for mating-type switching. Here we investigate whether this mutation confers any genome-wide defects. We show that the swi7-1 mutation renders cells hypersensitive to the DNA damaging agents methyl methansulfonate (MMS), hydroxyurea (HU) and UV and incapacitates activation of the intra-S checkpoint in response to DNA damage. In addition we show that, in the swi7-1 background, cells are characterized by an elevated level of repair foci and recombination, indicative of increased genetic instability. Furthermore, we detect novel Swi1-, -Swi3- and Pol α- dependent alkylation damage repair intermediates with mobility on 2D-gel that suggests presence of single-stranded regions. Genetic interaction studies showed that the flap endonuclease Fen1 works in the same pathway as Pol α in terms of alkylation damage response. Fen1 was also required for formation of alkylation- damage specific repair intermediates. We propose a model to explain how Pol α, Swi1, Swi3 and Fen1 might act together to detect and repair alkylation damage during S-phase.     

Involvement of Schizosaccharomyces pombe rrp1+ and rrp2+ in the Srs2- and Swi5/Sfr1-dependent pathway in response to DNA damage and replication inhibition

Previously we identified Rrp1 and Rrp2 as two proteins required for the Sfr1/Swi5-dependent branch of homologous recombination (HR) in Schizosaccharomyces pombe. Here we use a yeast two-hybrid approach to demonstrate that Rrp1 and Rrp2 can interact with each other and with Swi5, an HR mediator protein. Rrp1 and Rrp2 form co-localizing methyl methanesulphonate–induced foci in nuclei, further suggesting they function as a complex. To place the Rrp1/2 proteins more accurately within HR sub-pathways, we carried out extensive epistasis analysis between mutants defining Rrp1/2, Rad51 (recombinase), Swi5 and Rad57 (HR-mediators) plus the anti-recombinogenic helicases Srs2 and Rqh1. We confirm that Rrp1 and Rrp2 act together with Srs2 and Swi5 and independently of Rad57 and show that Rqh1 also acts independently of Rrp1/2. Mutants devoid of Srs2 are characterized by elevated recombination frequency with a concomitant increase in the percentage of conversion-type recombinants. Strains devoid of Rrp1 or Rrp2 did not show a change in HR frequency, but the number of conversion-type recombinants was increased, suggesting a possible function for Rrp1/2 with Srs2 in counteracting Rad51 activity. Our data allow us to propose a model placing Rrp1 and Rrp2 functioning together with Swi5 and Srs2 in a synthesis-dependent strand annealing HR repair pathway.     

S. pombe replication protein Cdc18 (Cdc6) interacts with Swi6 (HP1) heterochromatin protein: Region specific effects and replication timing in the centromere

Heterochromatin in S. pombe is associated with gene silencing at telomeres, the mating locus and centromeres. The compact heterochromatin structure raises the question how it unpacks and reforms during DNA replication. We show that the essential DNA replication factor Cdc18 (CDC6) associates with heterochromatin protein 1 (Swi6) in vivo and in vitro. Biochemical mapping and mutational analysis of the association domains show that the N-terminus of Cdc18 interacts with the chromoshadow domain of Swi6. Mutations in Swi6 that disrupt this interaction disrupt silencing and delay replication in the centromere. A mutation cdc18-I43A that reduces Cdc18 association with Swi6 has no silencing defect at the centromere, but changes Swi6 distribution and accelerates the timing of centromere replication. We suggest that fine tuning of Swi6 association at replication origins is important for negative as well as positive control of replication initiation.     

Swi1 and Swi3 are components of a replication fork protection complex in fission yeast

Swi1 is required for programmed pausing of replication forks near the mat1 locus in the fission yeast Schizosaccharomyces pombe. This fork pausing is required to initiate a recombination event that switches mating type. Swi1 is also needed for the replication checkpoint that arrests division in response to fork arrest. How Swi1 accomplishes these tasks is unknown. Here we report that Swi1 copurifies with a 181-amino-acid protein encoded by swi3(+). The Swi1-Swi3 complex is required for survival of fork arrest and for activation of the replication checkpoint kinase Cds1. Association of Swi1 and Swi3 with chromatin during DNA replication correlated with movement of the replication fork. swi1Delta and swi3Delta mutants accumulated Rad22 (Rad52 homolog) DNA repair foci during replication. These foci correlated with the Rad22-dependent appearance of Holliday junction (HJ)-like structures in cells lacking Mus81-Eme1 HJ resolvase. Rhp51 and Rhp54 homologous recombination proteins were not required for viability in swi1Delta or swi3Delta cells, indicating that the HJ-like structures arise from single-strand DNA gaps or rearranged forks instead of broken forks. We propose that Swi1 and Swi3 define a fork protection complex that coordinates leading- and lagging-strand synthesis and stabilizes stalled replication forks.     

A small, glutamine-free domain propagates the [SWI(+)] prion in budding yeast

Yeast prions are self-propagating protein conformations that transmit heritable phenotypes in an epigenetic manner. The recently identified yeast prion [SWI(+)] is an alternative conformation of Swi1, a component of the evolutionarily conserved SWI/SNF chromatin-remodeling complex. Formation of the [SWI(+)] prion results in a partial loss-of-function phenotype for Swi1. The amino-terminal region of Swi1 is dispensable for its normal function but is required for [SWI(+)] formation and propagation; however, the precise prion domain (PrD) of Swi1 has not been elucidated. Here, we define the minimal Swi1 PrD as the first 37 amino acids of the protein. This region is extremely asparagine rich but, unexpectedly, contains no glutamine residues. This unusually small prion domain is sufficient for aggregation, propagation, and transmission of the [SWI(+)] prion. Because of its unusual size and composition, the Swi1 prion domain defined here has important implications for describing and identifying novel prions.     

Swi1 Associates with Chromatin through the DDT Domain and Recruits Swi3 to Preserve Genomic Integrity

Swi1 and Swi3 form the replication fork protection complex and play critical roles in proper activation of the replication checkpoint and stabilization of replication forks in the fission yeast Schizosaccharomyces pombe. However, the mechanisms by which the Swi1-Swi3 complex regulates these processes are not well understood. Here, we report functional analyses of the Swi1-Swi3 complex in fission yeast. Swi1 possesses the DDT domain, a putative DNA binding domain found in a variety of chromatin remodeling factors. Consistently, the DDT domain-containing region of Swi1 interacts with DNA in vitro, and mutations in the DDT domain eliminate the association of Swi1 with chromatin in S. pombe cells. DDT domain mutations also render cells highly sensitive to S-phase stressing agents and induce strong accumulation of Rad22-DNA repair foci, indicating that the DDT domain is involved in the activity of the Swi1-Swi3 complex. Interestingly, DDT domain mutations also abolish Swi1's ability to interact with Swi3 in cells. Furthermore, we show that Swi1 is required for efficient chromatin association of Swi3 and that the Swi1 C-terminal domain directly interacts with Swi3. These results indicate that Swi1 associates with chromatin through its DDT domain and recruits Swi3 to function together as the replication fork protection complex.