The metK gene of Escherichia coli encodes S-adenosylmethionine synthetase, which catalyzes the reaction between L-methionine and ATP to form S-adenosylmethionine (SAM), the universal methyl group donor in the cell. SAM synthetase is generally assumed to be essential in E. coli , but this has not been proven. I demonstrated here that the metK gene could be deleted in E. coli by gene replacement only in the presence of a rescue plasmid carrying a functional copy of metK .  One commonly used metK mutant " metK84 " had a markedly reduced SAM synthetase level and exhibited a complex phenotype under certain growth conditions. Through DNA sequencing analysis and primer extension analysis, I showed that this mutant carried an A[arrow right]G transition in the -10 region of the metK promoter.  In this work, the initiation site for metK transcription was determined by primer extension. (Abstract shortened by UMI.)