Ribonucleoside Diphosphate Reductase Induced by Bacteriophage T4

Ribonucleoside diphosphate reductase induced in Escherichia coli by infection with bacteriophage T4 catalyzes the reduction of the 5'-diphosphates of cytidine, uridine, guanosine, and adenosine (CDP, UDP, GDP, and ADP). The specificity of the enzyme for the substrates is governed by different nucleoside triphosphates which act both by changing the maximal velocity of the reaction and the apparent Km for the substrates. Deoxyadenosine triphosphate (dATP) and ATP are the prime effectors for the reduction of the pyrimidine ribonucleotides CDP and UDP, deoxythymidine triphosphate (dTTP) is the prime effector for the reduction of GDP, and dGTP is the prime effector for the reduction of ADP. None of the nucleotides is a general inhibitor. Substrate saturation curves are hyperbolic both in the absence and presence of effector. In the presence of the prime effector the maximal velocity with each of the four substrates is of the same magnitude. The Km values for CDP, GDP, and ADP are about 0.3 mm in the absence of effector and about 0.05 mm in the presence of a prime effector. The corresponding Km values for UDP are 1.2 mm and 0.1 mm, respectively. It is suggested that the nucleoside triphosphates act by allosteric mechanisms. Competition experiments indicate that the enzyme contains a single allosteric site and a single catalytic site. The allosteric properties of T4 ribonucleotide reductase are in many ways similar to those of the E. coli enzyme. A main difference, however, concerns the effect of dATP. With the T4 enzyme this nucleotide is a prime effector for the reduction of pyrimidine ribonucleotides while it is an inhibitor for the reduction of all ribonucleotides with the E. coli enzyme.