A Deoxyribonucleic Acid Polymerase I-deficient Mutant of Bacillus subtiZis*

A methyl methanesulfonate sensitive mutant of Bacillus subtilis appears to lack the enzyme DNA polymerase I. The absence of polymerase I permits the detection of a second DNA polymerizing enzyme in the mutant. This second polymerase is inhibitable by sulfhydryl antagonists, and shows maximal activity on double-stranded DNA substrates with many single strand breaks. Poly[d(A-T)] is less efficiently used as a substrate, in contrast to the substrate preferences of polymerase I. Compared to the wild type, the mutant is more sensitive to ultraviolet and x-irradiation, and is inefficient in reactivation of ultraviolet-irradiated SPOl bacteriophage, but is normal in genetic recombination.