Abstract

The metK gene of Escherichia coli encodes for the enzyme S-adenosylmethionine (SAM) synthetase, which catalyzes the formation of S-adenosylmethionine. Isolated as a $\gamma$-glutamylmethyl ester-resistant mutant by Greene et al. (1973), a metK84 mutant grows normally on rich media (Greene et al. 1973; Lin et al., 1990). This mutation has been transferred via transduction to an $lrp\sp+$ background, where normal growth rates on minimal medium require the presence of 50 $\mu$g/ml L-leucine. When the leucine concentration is reduced to 25 $\mu$g/ml or less, cells fail to divide, resulting in the formation of long filaments. These filaments can undergo DNA replication, but fail to form crosswalls. SAM synthetase activity assays indicate that metK84 mutants maintain only 1.7% of the wild type activity when grown on minimal medium without added leucine. A mutation in lrp, or the addition of leucine increases metK expression in both wild type and mutant strains. When metK activity is increased to a level of 0.26 mmol/min/mg of protein (11.3% of wild type activity), the mutant strain undergoes normal growth and cell division. The wild type metK gene has been cloned, and expression of the plasmid-borne gene complements for the growth-deficiency phenotypes of metK84. However, when the plasmid is over-expressed, the culture becomes auxotrophic for methionine. This work indicates that a SAM deficiency results in a cell division defect in E. coli.