Evidence that DNA polymerase-alpha of calf thymus contains a subunit of molecular weight 155000.

Small samples of the 8-S species of enzymes (A1 and A2) occurring in the DNA polymerase-alpha fraction of calf thymus, have been extensively purified using non-denaturing (normal) polyacrylamide gel electrophoresis. When peak fractions of activity on normal gels were subjected to dodecylsulphate-polyacrylamide gel electrophoresis, a polypeptide at 155000 correlated with polymerase activity. Samples of the 7.3-S (C) enzyme prepared from A2 by treatment with 2.4 M urea or isolated directly without exposure to urea, also showed the presence of a 155000-Mr polypeptide. It is concluded that the 7.3-S (C) enzyme, of previously estimated molecular weight 155000-170000, is a single polypeptide and that the 8-S enzymes A1 and A2 contain an additional subunit of 50000-70000 molecular weight.