Effects of T4 phage infection and anaerobiosis upon nucleotide pools and mutagenesis in nucleoside diphosphokinase-defective Escherichia coli strains

Abstract

Bacteriophage T4 encodes nearly all of its own enzymes for synthesizing DNA and its precursors. An exception is nucleoside diphosphokinase (ndk gene product), which catalyzes the synthesis of ribonucleoside triphosphates and deoxyribonucleoside triphosphates (dNTPs) from the corresponding diphosphates. Surprisingly, an Escherichia coli ndk deletion strain grows normally and supports T4 infection. As shown elsewhere, these ndk mutant cells display both a mutator phenotype and deoxyribonucleotide pool abnormalities. However, after T4 infection, both dNTP pools and spontaneous mutation frequencies are near normal. An E. coli strain carrying deletions in ndk and pyrA and pyrF, the structural genes for both pyruvate kinases, also grows and supports T4 infection. We examined anaerobic E. coli cultures because of reports that in anaerobiosis, pyruvate kinase represents the major route for nucleoside triphosphate synthesis in the absence of nucleoside diphosphokinase. The dNTP pool imbalances and the mutator phenotype are less pronounced in the anaerobic than in the corresponding aerobic ndk mutant strains. Anaerobic dNTP pool data, which have not been reported before, reveal a disproportionate reduction in dGTP, relative to the other pools, when aerobic and anaerobic conditions are compared. The finding that mutagenesis and pool imbalances are mitigated in both anaerobic and T4-infected cultures provides strong, if circumstantial, evidence that the mutator phenotype of ndk mutant cells is a result of the dNTP imbalance. Also, the viability of these cells indicates the existence of a second enzyme system in addition to nucleoside diphosphokinase for nucleoside triphosphate synthesis.