Bacteriophage T3- and T7-directed deoxyribonucleases

The net content of deoxyribonucleic acid (DNA) of Escherichia coli cells infected with the wild types of T3 or T7 does not change markedly throughout the latent period (A. B. Pardee and I. Williams, Ann. Inst. Pasteur 84:147, 1953; F. W. Putnam et al., J. Biol. Chem. 199:177, 1952). However, we recently described the breakdown of host DNA by "early" amber mutants of two complementation groups of both T3 and T7 (R. Hausmann and B. Gomez, J. Virol. 1:779, 1967). These combined observations suggest that, upon infection with the wild types of these phages, breakdown of host DNA also occurs but is masked by the simultaneous buildup of phagespecific DNA. Thus, the question arises whether this breakdown is related to the synthesis of phage-specific nucleases or whether phage infection renders the host DNA accessible to preexisting nucleases of the host. We think that this latter view is favored by the findings of A. B. Pardee and I. Williams (Ann. Inst. Pasteur 84: 147, 1953), who reported a relatively slight increase in deoxyribonuclease activity in host cells after infection with T3. We used different assay conditions and found that pronounced increases in deoxyribonuclease activity occurred shortly after infection with phage T3 or T7. To assay deoxyribonuclease activity in extracts of phage-infected E. coli B, we determined the amount of radioactivity released in acid-soluble form from 3H-thymidine-labeled E. coli B DNA (A. Weissbach and D. Korn, J. Biol. Chem. 238:3383, 1963). Procedures of phage-infection, harvesting of cells, and preparation of cell-free extracts were similar to those described by M. Gold et al. (Proc. Natl. Acad. Sci. U.S. 52:292, 1964). Assays were first conducted with incubation mixtures buffered at various pH values, and both native and alkali-denatured DNA preparations were used as substrates. We used extracts of cells harvested 10 min after infection with T3 or T7 as an enzyme source. Relative to uninfected cells, maximal increases in deoxyribonuclease activity, as compared to native DNA, were about 1.3-fold for T3-infected cells and about fourfold for T7-infected cells; as compared to denatured DNA, the relative increases were about fourand sixfold, respectively (Fig. 1). Similar results were