Isolation of an altered form of DNA polymerase I from Escherichia coli cells induced for recA/lexA functions

A novel form of DNA polymerase I (deoxynucleosidetriphosphate:DNA deoxynucleotidyltransferase, DNA nucleotidyltransferase, EC 2.7.7.7) activity has been isolated from Escherichia coli cells that had been activated for expression of the DNA damage-inducible genes. Induction was by treatment of normal cells or cells carrying the spr-51 and tif-1 mutations with nalidixic acid. This activity, DNA polymerase I, seems to be a form of DNA polymerase I because it is insensitive to N-ethylmaleimide, is inhibited by antibody to DNA polymerase I, and does not appear in a polA1 strain. DNA polymerase I activity sediments through sucrose gradients as a broad peak with s20.w = 6.6--10.5, compared with an s20,w = 4.8--5.5 for DNA polymerase I. The fidelity during polymerization reactions of DNA polymerase I is relatively low with a variety of synthetic templates and deoxynucleoside triphosphates, although the enzyme appears to have a normal level of 3' greater than 5' exonuclease. Polymerase I has properties that might implicate it in some form of mutagenic DNA repair.