Studies on a mutant strain of Escherichia coli which requires both methionine and lysine for growth.

A mutant of Esckch ia coZi waa isolated which showed a requirement for methionine + lysine when a fairly large inoculum of washed organisms was used. Cystathionine or homocysteine replaced methionine, and diaminopimelic acid replaced lysine; no other amino acid or growth factor was active. The amounts of the amino acids required to promote growth were only about one-tenth of the quantity of methionine and lysine found in the grown organisms. A quantitative study with lV-glucose and 85S-sulphate confirmed that these amino acids were synthesized de novo. Although homoserine would not replace methionine for growth, the presence of unlabelled homoserine suppressed the incorporation of isotope from 14C-glucose into methionine and threonine, indicating that the accepted pathways are operative and that the block in methionine synthesis was in the formation of cystathionine. Small amounts of yeast extract replaced the amino acid requirement and decreased one-thousandfold the size of the inoculum required.