Bacteriophage T4 ribonucleoside diphosphate reductase: on the defect causing decreased formation of the beta 93(2) subunit encoded by the nrdB93 mutant gene.

Bacteriophage T4 ribonucleoside diphosphate reductase is composed of two proteins, alpha 2 and beta 2, encoded by the nrdA and nrdB genes, respectively. The expression of nrdB is the limiting factor for the assembly of the enzyme. A recently described mutation, nrdB93, may give new insight into the regulation of synthesis of the beta subunit encoded by nrdB. Infection by T4 nrdB93 produced only low concentrations of the beta 2(93) protein. However, a site-specific mutation of phage T4 gene 39, encoding one of the subunits of T4 DNA topoisomerase, phenotypically suppressed the defect. The present work sought to characterize the nature of this defect. The mutation in nrdB93 was a single-base transition (G-->A) resulting in a Gly253-->Asp change. In vivo and in vitro studies provided no evidence of degradation of the beta 2(93) protein. Furthermore, the decrease in beta 2(93) formation was not caused by a delayed onset of transcription, neither by a decreased rate of mRNA formation from the nrdB promoter, nor by a defective intron splicing of the nrdB gene or in the transcription of the terminal segments of the message. These findings are consistent with the concept that the nrdB93 lesion produces a defect at the level of translation.