Bacteriophage T4 infection is known to induce the formation of a complex of enzymes effecting the de novo synthesis of deoxyribonucleoside triphosphates, which in turn are channeled into T4 DNA replication. The first step in this pathway is catalyzed by a ribonucleoside diphosphate reductase, comprised of subunits coded by T4 genes nrdA and nrdB. Maximum rates of synthesis of the pyrimidine deoxyribonucleotides and of DNA replication in vivo also require a type II DNA topoisomerase encoded by T4 genes 39, 52, and 60. We report the identification of a unique mutant, nrdB93, and the suppression of its defective deoxyribonucleotide synthesis by a gene 39 mutation, 39-01. After infection by 39-01, DNA synthesis and plaque formation were temperature-sensitive, but nearly wild type rates of deoxyribonucleotide synthesis were retained at all temperatures. The nrdB93 mutation had a profound effect on deoxyribonucleotide synthesis at 41 degrees C; even at the permissive temperature of 30 degrees C, synthesis was reduced to 30% of that of wild type or 39-01. However, on infection at 30 degrees C by the double mutant, 39-01 nrdB93, the level of deoxyribonucleotide synthesis again reached that of wild type phage infections; involvement of the comparable host enzyme in the suppression process has been excluded. Suppression of the effect of nrdB93 by 39-01 implicates the gene 39 product in the regulation of nrdB expression. The accompanying paper (Cook, K. S., Wirak, D. O., Seasholtz, A. F., and Greenberg, G. R. (1988) J. Biol. Chem. 263, 6202-6208) examines the nature of the suppression process at the molecular level.