Integrating Sister Chromatid Cohesion Establishment to DNA Replication.

The intersection through which two fundamental processes meet provides a unique vantage point from which to view cellular regulation. On the one hand, DNA replication is at the heart of cell division, generating duplicate chromosomes that allow each daughter cell to inherit a complete copy of the parental genome. Among other factors, the PCNA (proliferating cell nuclear antigen) sliding clamp ensures processive DNA replication during S phase and is essential for cell viability. On the other hand, the process of chromosome segregation during M phase-an act that occurs long after DNA replication-is equally fundamental to a successful cell division. Eco1/Ctf7 ensures that chromosomes faithfully segregate during mitosis, but functions during DNA replication to activate cohesins and thereby establish cohesion between sister chromatids. To achieve this, Eco1 binds PCNA and numerous other DNA replication fork factors that include MCM helicase, Chl1 helicase, and the Rtt101-Mms1-Mms22 E3 ubiquitin ligase. Here, we review the multi-faceted coordination between cohesion establishment and DNA replication. SUMMARY STATEMENT: New findings provide important insights into the mechanisms through which DNA replication and the establishment of sister chromatid cohesion are coupled.

PCNA antagonizes cohesin-dependent roles in genomic stability.

PCNA sliding clamp binds factors through which histone deposition, chromatin remodeling, and DNA repair are coupled to DNA replication. PCNA also directly binds Eco1/Ctf7 acetyltransferase, which in turn activates cohesins and establishes cohesion between nascent sister chromatids. While increased recruitment thus explains the mechanism through which elevated levels of chromatin-bound PCNA rescue eco1 mutant cell growth, the mechanism through which PCNA instead worsens cohesin mutant cell growth remains unknown. Possibilities include that elevated levels of long-lived chromatin-bound PCNA reduce either cohesin deposition onto DNA or cohesin acetylation. Instead, our results reveal that PCNA increases the levels of both chromatin-bound cohesin and cohesin acetylation. Beyond sister chromatid cohesion, PCNA also plays a critical role in genomic stability such that high levels of chromatin-bound PCNA elevate genotoxic sensitivities and recombination rates. At a relatively modest increase of chromatin-bound PCNA, however, fork stability and progression appear normal in wildtype cells. Our results reveal that even a moderate increase of PCNA indeed sensitizes cohesin mutant cells to DNA damaging agents and in a process that involves the DNA damage response kinase Mec1(ATR), but not Tel1(ATM). These and other findings suggest that PCNA mis-regulation results in genome instabilities that normally are resolved by cohesin. Elevating levels of chromatin-bound PCNA may thus help target cohesinopathic cells linked that are linked to cancer.

PCNA promotes context-specific sister chromatid cohesion establishment separate from that of chromatin condensation.

Cellular genomes undergo various structural changes that include cis tethering (the tethering together of two loci within a single DNA molecule), which promotes chromosome condensation and transcriptional activation, and trans tethering (the tethering together of two DNA molecules), which promotes sister chromatid cohesion and DNA repair. The protein complex termed cohesin promotes both cis and trans forms of DNA tethering, but the extent to which these cohesin functions occur in temporally or spatially defined contexts remains largely unknown. Prior studies indicate that DNA polymerase sliding clamp PCNA recruits cohesin acetyltransferase Eco1, suggesting that sister chromatid cohesion is established in the context of the DNA replication fork. In support of this model, elevated levels of PCNA rescue the temperature growth and cohesion defects exhibited by eco1 mutant cells. Here, we test whether Eco1-dependent chromatin condensation is also promoted in the context of this DNA replication fork component. Our results reveal that overexpressed PCNA does not promote DNA condensation in eco1 mutant cells, even though Smc3 acetylation levels are increased. We further provide evidence that replication fork-associated E3 ligase impacts on Eco1 are more complex that previously described. In combination, the data suggests that Eco1 acetylates Smc3 and thus promotes sister chromatid cohesion in context of the DNA replication fork, whereas a distinct cohesin population participates in chromatin condensation outside the context of the DNA replication fork.

Genome Sequences of Six Cluster N Mycobacteriophages, Kevin1, Nenae, Parmesanjohn, ShrimpFriedEgg, Smurph, and SpongeBob, Isolated on Mycobacterium smegmatis mc2155.

The annotation of six cluster N Mycobacterium smegmatis phages (Kevin1, Nenae, Parmesanjohn, ShrimpFriedEgg, Smurph, and SpongeBob) reveals regions of genomic diversity, particularly within the central region of the genome. The genome of Kevin1 includes two orphams (genes with no similarity to other phage genes), with one predicted to encode an AAA-ATPase.

Genome Sequences of Chancellor, Mitti, and Wintermute, Three Subcluster K4 Phages Isolated Using Mycobacterium smegmatis mc2155.

Mycobacteriophages Chancellor, Mitti, and Wintermute infect Mycobacterium smegmatis mc2155 and are closely related to phages Cheetobro and Fionnbharth in subcluster K4. Genome sizes range from 57,697 bp to 58,046 bp. Phages are predicted to be temperate and to infect the pathogen Mycobacterium tuberculosis.