During the infection by bacteriophage T4, dTMP and 5hydroxymethyl dCMP (HmdCMP) are synthesized in the ratio of 2:1, respectively, i.e. at the Thy/HmCyt ratio found in T4 DNA. This study demonstrates that on infection by a cd (phage-induced dCMP deaminase) mutant the dTMP/ HmdCMP synthesis ratio is approximately 0.6. The ratio remains about 0.6 on infection by cd dna double mutants. At the same time, with cd the combined rate of dTMP plus HmdCMP synthesis increases by about 50% over the wild type rate. The rates of synthesis of dTMP and of DNA with phage cd appear to remain the same as with phage T4. However, free HmdCMP derivatives are greatly overproduced and accumulate. From these experiments, it has been estimated that in wild type phage infections approximately 40% of the dTMP derives from dCMP. Treatment of a T4D-infected culture by proflavine 10 min after infection blocks the synthesis of both dTMP and HmdCMP, but eventually the ratio of dTMP/HmdCMP synthesis increases above 2:l. With cultures infected by cd, the synthesis of dTMP and HmdCMP is also inhibited, but the ratio does not increase above O.&l. Thus the rise in ratio which proflavine causes in wild type phage infection requires the dCMP deaminase pathway. These experiments demonstrate that dCMP deaminase plays an important quantitative role in maintaining the correct ratio of dTMP/HmdCMP synthesis. Regulation of the ratio at 2:l depends on factors intrinsic to the complex of enzymes synthesizing deoxyribonucleotides (Flanegan, J. B., and Greenberg, G. R. (197715. Biol. Chem. 252, 30193027).
C. Chiu, T. Ruettinger, J. Flanegan
The Journal of biological chemistry