The DNA-protein complex which forms in the cytoplasm of HeLa cells during the course of vaccinia virus infection contains a single DNA-binding phosphoprotein, termed FP11.This phosphoprotein was purified to near homogeneity by chromatog. on denatured DNA-cellulose.Phosphoprotein FP11 has an apparent mol. weight of 34,000, as determined by electrophoresis in polyacrylamide gels containing Na dodecyl sulfate.High-voltage paper electrophoresis following acid hydrolysis reveals that the polypeptide contains phosphothreonine but no phosphoserine.The in vivo phosphorylation occurs at a min. of 4 distinct phosphothreonine residues.FP11 is an acidic protein with an isoelec. point lying in the pH region 5.2-5.5.