Deoxycytosine methylase (Dcm) enzyme activity causes mutagenesis in vitro either 
directly by enzyme-induced deamination of cytosine to uracil in the absence of 
the methyl donor, S-adenosylmethionine (SAM), or indirectly through spontaneous 
deamination of [5-methyl]cytosine to thymine. Using a Lac reversion assay, we 
investigated the contribution of the first mechanism to Dcm mutagenesis in vivo 
by lowering the levels of SAM. Escherichia coli SAM levels were lowered by 
reducing SAM synthetase activity via the introduction of a metK84 allele or by 
hydrolyzing SAM using the bacteriophage T3 SAM hydrolase. The metK84 strains 
exhibited increased C-to-T mutagenesis. Expression of the T3 SAM hydrolase gene, 
under the control of the arabinose-inducible P(BAD) promoter, effectively 
reduced Dcm-mediated genomic DNA methylation. However, increased mutagenesis was 
not observed until extremely high arabinose concentrations were used, and genome 
methylation at Dcm sites was negligible.