Allosteric effectors are required for subunit association in T4 phage ribonucleotide reductase

Bacteriophage T4 encodes its own aerobic ribonucleotide reductase (RNR), which reduces ribonucleoside diphosphates to the corresponding deoxyribonucleoside diphosphates. T4 RNR is composed of homodimeric large (R1) and small (R2) subunits. Intricate regulation of enzymatic activity is accomplished by the binding of nucleotide effectors to R1. Berglund (Berglund, O. (1972) J. Biol. Chem. 247, 7270-7275) described similarities between T4 RNR and the corresponding enzyme from aerobic Escherichia coli. An important difference, however, is that T4 RNR forms a tight R1.R2 complex, while the E. coli R1 and R2 more readily dissociate. In this study we purified the phage R2 subunit from an overexpression vector constructed by Tseng et al. (Tseng, M., Hilfinger, J., He, P., and Greenberg, R. (1992) J. Bacteriol. 174, 5740-5744) and used this as an immunogen to generate polyclonal antiserum. Using co-immunoprecipitation techniques, we probed in vitro for interactions between the phage-induced R1 and R2 subunits. Our studies indicate that tight binding of the phage RNR subunits is completely dependent upon the known allosteric effectors of the enzyme. Once the R1.R2 holoenzyme has been formed it appears to be remarkably stable when in the presence of dATP. However, if dATP is removed, the R1.R2 complex readily dissociates.