SUMMARY: The observation that growth in the presence of methionine results in organisms with a greatly decreased ability to form methionine from homocysteine has been extended to other strains of , in particular strain PA 15, an organism for which there is a considerable background of knowledge concerning the enzymic mechanism of this stage of methionine synthesis. When organisms grown in the absence of methionine (‘active’ organisms) are transferred to a medium containing methionine the activity is not lost, but simply diluted by new organisms having much less activity. Both organisms and ultrasonic extracts derived from them show the same comparative loss of activity when the organisms are incubated with methionine.

Organisms grown in the presence of methionine (‘inactive’ organisms) regain activity when incubated in a growth medium without methionine. The major part of such regeneration of activity occurs before growth is significant. Enzyme formation occurs linearly and without lag and is stopped by the addition of methionine at any stage. A number of analogues of methionine also repress enzyme formation, but none is as active as methionine.

Regeneration of enzyme is inhibited by chloramphenicol and is greatly diminished when the supply of individual amino acids is restricted either by the use of an inhibitory analogue (-fluorophenylalanine) or by the use of amino acid-requiring auxotrophs. Formation of enzyme therefore requires synthesis of protein. Similar techniques gave no evidence that concurrent synthesis of ribonucleic acid was also required.

Compounds which act as substrates or cofactor for methionine synthesis (serine, homocysteine, cobalamin) also repressed enzyme formation to some extent and, after a lag. full repression was obtained with a mixture of all three. These effects are probably due to methionine itself whose synthesis by the enzyme initially present is stimulated by the test substances.


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