Effect of 5-azacytidine on DNA methylation and on the enzymes of de novo pyrimidine biosynthesis in Bacillus subtilis Marburg strain.

In Bacillus subtilis, 5-azacytidine, an analog of cytidine, causes a time- and dose-dependent growth inhibition. Methyl donors are unable to overcome azacytidine-induced inhibition while pyrimidine nucleosides, except orotidine, can revert this inhibition totally. On the other hand, pyrimidine bases, except uracil, are unable to restore growth in azacytidine-treated cells. Uracil, at a high concentration, can revert growth inhibition only inefficiently. However, a considerable relief of growth inhibition by uracil occurs in the presence of a ribose donor. In azacytidine-treated B. subtilis cells methylation of bases in DNA is not affected either quantitatively or qualitatively and DNA methyltransferase activity remains unaltered as compared to the untreated cells, apparently due to the absence of azacytidine incorporation into the DNA. The inability of B. subtilis cytidine kinase to phosphorylate azacytidine is the probable reason for this non-incorporation. Analysis of the enzymes of de novo pyrimidine biosynthesis has shown that orotidine monophosphate pyrophosphorylase is specifically repressed by azacytidine treatment.