BACTERIOPHAGE-T4 RIBONUCLEOSIDE DIPHOSPHATE REDUCTASE - ON THE DEFECTCAUSING DECREASED FORMATION OF THE BETA(93)(2) SUBUNIT ENCODED BY THENRDB93 MUTANT-GENE

Bacteriophage T4 ribonucleoside diphosphate reductase is composed of t wo proteins, alpha(2) and beta(2), encoded by the nrdA and nrdB genes, respectively. The expression of nrdB is the limiting factor for the a ssembly of the enzyme. A recently described mutation, nrdB93, may give new insight into the regulation of synthesis of the beta subunit enco ded by nrdB. Infection by T4 nrdB93 produced only low concentrations o f the beta(2)(93) protein. However, a site-specific mutation of phage T4 gene 39, encoding one of the subunits of T4 DNA topoisomerase, phen otypically suppressed the defect. The present work sought to character ize the nature of this defect. The mutation in nrdB93 was a single-bas e transition (G-->A) resulting in a Gly(253)-->Asp change. In vivo and in vitro studies provided no evidence of degradation of the beta(2)(9 3) protein. Furthermore, the decrease in beta(2)(93) formation was not caused by a delayed onset of transcription, neither by a decreased ra te of mRNA formation from the nrdB promoter, nor by a defective intron splicing of the nrdB gene or in the transcription of the terminal seg ments of the message. These findings are consistent with the concept t hat the nrdB93 lesion produces a defect at the level of translation.