Enzymatic synthesis of deoxy-5-methyl-cytidylic acid replacing deoxycytidylic acid in Xanthomonas oryzae phage Xp12 DNA

Abstract

PHAGE Xp12 on Xanthomonas oryzae was isolated from the irrigation water of a rice field. In the DNA of this phage, cytosine is completely replaced by 5-methylcytosine¹. Isotope tracing studies on the biosynthesis of this unusual pyrimidine demonstrated that methylation of this cytosine is different from that of the 5-methylcytosine which is found in trace amounts in most plant and animal DNAs. The latter is methylated by methionine. On the other hand, the methyl group of 5-methylcytosine residues of Xp12 is derived from the 3-carbon of serine and not from the thiomethyl carbon of methionine². In this investigation, the synthesis of deoxy-5-methylcytidylic acid (d5MCMP) was found to take place at the level of nucleotide, with deoxycytidylic acid (dCMP) and formaldehyde as substrates in the presence of 5,6,7,8-tetrahydrofolic acid (THFA) and an enzyme preparation from X. oryzae infected with Xp12 phage. This reaction seems to account for the synthesis of the unique pyrimidine, 5-methylcytosine, which occurs in the DNA of phage Xp12.