Constitutive inhibition of RecBCD-mediated DNA degradation in the Escherichia coli K-12 radiation-resistant mutant Gam(r)444

Exonucleolytic degradation of [H-3]-labeled DNA was examined in partially p urified fractions of lysates obtained from nonirradiated RecBCD enzyme-cont aining cells of Escherichia coil and in the radiation-resistant mutant Gam( r)444. The degradative activity was shown to be lowered in these cells to t he same extent as in the recBC mutant. The efficiency of plating of the mut ant phage T4 2(-), DNA of which can be degraded by exonuclease V, was 400-f old higher on the strain Gam(r)444 than on the wild-type strain AB1157. Thi s value was shown to be only twice as low as that on the recB mutant or on the strain AB1157 carrying plasmid pGam26 with a radiation-resistance allel e gam26 cloned from mutant Gam(r)444. The data obtained confirmed the hypot hesis that the Gam(r)444 mutant contains a constitutive inhibitor of exonuc leolytic activity of the RecBCD enzyme in nonirradiated cells. This inhibit or was shown to be encoded by the gam26 allele that had previously been map ped at 56.8 min of the E. coli chromosome. A possible mechanism of the invo lvement of this inhibitor in enhanced radiation resistance of the mutant Ga m(r)444 is considered.