Sliding clamps are loaded onto DNA by clamp loaders to serve thecritical role of coordinating various enzymes on DNA. Clamp loadersmust quickly and efficiently load clamps at primer/template (p/t)junctions containing a duplex region with a free 3′OH (3′DNA), butit is unclear how clamp loaders target these sites. To measure theEscherichia coli and Saccharomyces cerevisiae clamp loaderspecificity toward 3′DNA, fluorescent β and PCNA clamps were usedto measure clamp closing triggered by DNA substrates of differingpolarity, testing the role of both the 5′phosphate (5′P) and thepresence of single-stranded binding proteins (SSBs). SSBs inhibitclamp loading by both clamp loaders on the incorrect polarity ofDNA (5′DNA). The 5′P groups contribute selectivity to differingdegrees for the two clamp loaders, suggesting variations in themechanism by which clamp loaders target 3′DNA. Interestingly, the χsubunit of the E. coli clamp loader is not required for SSB toinhibit clamp loading on phosphorylated 5′DNA, showing that χ·SSBinteractions are dispensable. These studies highlight a common rolefor SSBs in directing clamp loaders to 3′DNA, as well as uncovernuances in the mechanisms by which SSBs perform this vitalrole.”

https://academic.oup.com/nar/article/42/16/10655/2903350

(5 points) What is a sliding clamp and what is its function?

(5 points) What is a sliding clamp loader and how does it aid inthe function of the sliding clamp?

(5 points) What is the significance of a recessed 3’-end DNA,during the p/t discrimination? What data and figure from the NARpaper, suggest this significance?

(5 points) What do you think would happen if the sliding clampcould not differentiate between 3’-OH and 5’-P ends of DNA? Howwould this effect DNA replication?