Pathways of pyrimidine deoxyribonucleotide biosynthesis in Mycoplasma mycoides subsp. mycoides

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Abstract
By measuring the specific activity of deoxyribonucleotides isolated from DNA after the incorporation of 14C-labeled precursors with and without competition from other nucleotide precursors, the major pathways of pyrimidine deoxyribonucleotide synthesis in M. mycoides ssp. mycoides were defined. Uracil, guanine and thymine are required for the synthesis of nucleotides. Cytidine competed effectively with uracil to provide all of the deoxycytidine nucleotide, as well as most of the deoxyribose-1 phosphate, for the synthesis of thymidylate from thymine via thymidine phosphorylase. Each of dUMP, dCMP and dTMP competed with cytidine for incorporation into DNA thymidylate. Appreciable incorporation of exogenous deoxyribonucleoside 5''-monophosphates into DNA without prior dephosphorylation was observed. Dephosphorylation also occurred since the added deoxyribonucletoide provide phosphate for the synthesis of the other nucleotides in DNA in competition with the 32Pi in the growth medium. Hydroxyurea inhibited cell growth and decreased the intracellular level of dATP, consistent with the action of a ribonucleoside diphosphate reductase with regulatory properties similar to those of the Escherichia coli enzyme.

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