Properties of metK mutants of Escherichia coli K-12

Some of the properties of three metK mutants of Escherichia coli K-12 have been examined. All three strains have lower than normal levels of SAM (S-adenosyl-l-methionine) synthetase and elevated levels of cystathionine synthetase and cystathionase. One strain (RG73) appears to have an unstable SAM synthetase, suggesting that it carries a structural gene mutation. The two strains (RG62 and RG109) which have the lowest levels of SAM synthetase when grown on minimal medium have appreciably higher levels of enzyme when grown on complete medium. Growth on defined media supplemented with leucine or methionine causes a several-fold increase in the specific activity of SAM synthetase with associated decreases in cystathionine synthetase and cystathionase, but the changes are not as large as those seen in cells grown on LB broth. The SAM pools of strains RG62 and RG109 are markedly lower than normal while that of strain RG73 is slightly below normal. The methionine pools of all three strains are elevated several-fold. The metK strains are able to synthesize cyclopropane fatty acids, but the rate of their formation is slowed. Modification and restriction of phage 21 appears to be normal, suggesting that these strains are able to methylate DNA.