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There are three mutations (recA, zab, and lexA) each of which suppresses the expression of the Escherichia coli tif mutation and causes high deoxyribonucleic acid (DNA) repair deficiency (Castellazi et al., 1972). The effect of the zab mutation on DNA stability was investigated. In vivo, a strain carrying the zab-53 mutation shows (i) no spontaneous DNA degradation and (ii) rapid DNA degradation after ultraviolet irradiation, which depends upon the exonuclease V activity coded by recB+/C+genes and which is independent from the correndonuclease II activity coded by uvrA+/B+. Thus, in regard to DNA stability, the zab mutant behaves like lexA and recA (Howard-Flanders and Boyce, 1966), the latter mutant showing in addition spontaneous DNA breakdown. The degradation patterns of these tif-suppressed strains are shown to be remarkably reproducible in bacteria made permeable to metabolites, by toluene or toluene plus Triton X-100. The degradation properties reflect the activity of the same biochemical system that works in vivo, in that degradation depends upon the presence of recA, zab, or lexA, ultraviolet irradiation, and exonuclease V activity. In addition, adenosine 5'-triphosphate (1 mM) is required. This assay with permeabilized cells offers a useful tool for studying degradation under controlled conditions, especially by permitting the dissociation of energy-dependent from energy-independent steps.

Deoxyribonucleic acid degradation in vivo and in permeabilized Escherichia coli repair-deficient (recA zab lexA) derivatives