PHOSPHORYLATION AND OTHER PROPERTIES OF PROTEINS THAT BIND TO SINGLE-STRANDED-DNA (SSB-PROTEINS) FROM CHROMATIN AND THE EXTRACHROMATIN FRACTION OF EHRLICH ASCITES-CARCINOMA CELLS

A comparative investigation was made of the properties of the proteins that bind to single-stranded DNA (SSB-proteins), isolated from chromatin and the extrachromatin fraction of Ehrlich ascites carcinoma cells. No differences were detected in the electrophoretic mobility under denaturing conditions, the ability to bind single-stranded DNA and to stimulate replicative DNA synthesis. However, the SSB-proteins of the chromatin contained 40-50% more phosphate than the extrachromatin proteins. Both preparations were phosphorylated in vitro by protein kinase C and cAMP-dependent protein kinase; the chromatin proteins were phosphorylated to a lesser degree. Phosphorylation was paralleled by an increase in the strength of binding of the SSB-proteins to single-stranded DNA-cellulose. Phosphorylation can regulate the functions of the SSB proteins by changing their affinity for chromatin DNA.