Ligase-Defective Bacteriophage T4 II. Physiological Studies

Abstract

The timing of the suppression of gene 30 (deoxyribonucleic acid ligase) mutations by rII mutations was studied by temperature shift-down experiments with a temperature-sensitive rII mutation. The rII function must remain inactivated for about 5 to 8 min at 37 C for suppression to occur, thus making suppression an early function. This result is in agreement with the timing of expression of other rII functions. A gene 30 defect can also be overcome by replacing the Na + cation in the growth medium with the Mg 2+ cation, a result similar to the relief of the lethality of rII mutations in lambda lysogens. Prior infection with bacteriophages T3 or T7, which produce their own deoxyribonucleic acid ligases, can also partially overcome the lethality of gene 30 mutations.

Ligase-Defective Bacteriophage T4 I. Effects on Mutation Rates

“…Selection artifacts are very improbable in all of these proflavine experiments. The frequency of rII+ revertants induced by proflavine is generally very much higher than the input of preexisting spontaneous revertants, and experiments reported in the following paper (13) show that the gene 30-rH interaction is complete by the time proflavine is added, so newly induced rII+ revertants are unlikely to reintroduce selection against the ts gene 30 mutation.…”

“…Although the T4rII system is usually excellent for measuring mutation rates, it has an important shortcoming for studies involving gene 30: rII mutations suppress the lethality of gene 30 mutations (3,13). T4rII mutations apparently reduce the amount of ligase needed for phage growth, either directly or indirectly decreasing endonuclease activity or else freeing host ligase for action on phage DNA (11,12).…”