Bacteriophage T4 inhibits colicin E2-induced degradation of Escherichia coli DNA : III. Zone sedimentation analyses of the DNA degradation products

Abstract

Zone sedimentation analyses of DNA degradation products have been used to determine directly which stages of colicin E2-induced DNA degradation are subject to inhibition by infection with bacteriophage T4. Infection with a T4 mutant (gene denA) deficient in an early stage of T4-induced degradation (endonuclease II, which nicks, i.e. breaks one strand of a double strand) inhibits the breakage of double strands (cutting) normally induced by colicin E2. The action of T4 polynucleotide ligase is not solely responsible (if at all) for this inhibition, because a T4 mutant deficient in both endonuclease II and ligase still inhibits E2-induced cutting activity. In control experiments in which E2 was omitted, the T4 mutant deficient in endonuclease II and ligase (and deoxycytidylate hydroxymethylase, to prevent phage DNA synthesis) cuts and nicks host DNA more extensively than the corresponding mutant having a functional ligase gene. Infection with a T4 mutant (gene 46) deficient in a late stage of phage-induced DNA degradation (solubilization of 10 6 molecular weight fragments) prevents the E2-induced solubilization of host DNA and leaves this DNA in fragments of 10 6 average molecular weight. We conclude that, under conditions that permit protein synthesis, T4 infection inhibits both the cutting and solubilizing activities of E2; it is not clear whether E2 alone causes nicking under our conditions and thus whether T4 inhibits such nicking. When protein synthesis is blocked by chloramphenicol, T4 infection has virtually no effect on the E2-induced production of 10 6 molecular weight fragments of DNA, but does inhibit their solubilization; however, this inhibition is less complete than when protein synthesis is permitted.