Effect of T4 modification of host valyl-tRNA synthetase on enzyme action in vivo

Abstract

It has previously been found that bacteriophage T4 modifies the valyl-tRNA synthetase of its host; in this work we have examined the effects of this modification on the action of the enzyme in vivo. For this purpose we compared the properties of T4-infected cells with those of uninfected cells or cells infected with the amber mutant vs2, which by all available criteria seems to leave the host enzyme completely unmodified. We find that, although the overall rate of transfer of valine into protein is reduced after infection, modification is apparently not the cause of this reduction. Also, modification does not alter the level of aminoacylation of the total valine tRNA pool nor of the invidividual species of valine tRNA. Nor does modification appear to be necessary to maintain these levels after infection. We found no evidence that modification affects any unknown valyl-tRNA synthetase reaction that yields novel valine-containing products. Although the modified enzyme is more stable in vitro than the unmodified enzyme, modification does not seem to facilitate phage growth at temperatures above the optimum. To all appearances, therefore, T4-directed modification of the host valyl-tRNA synthetase does not detectably affect the in vivo catalytic action of the enzyme in any straightforward way, at least under prevailing laboratory conditions.