SUMMARY: The amounts of cystathionine-forming enzymes (homoserine -trans-succinylase and cystathionine synthetase) of were decreased by growth with methionine, and therefore enzymic repression applied to all the enzymes of the methionine biosynthetic pathway. Enzyme resynthesis occurred when ‘inactive’ organisms (organisms grown with methionine) were suspended in methionine-free culture fluid. Although the formation of all the enzymes of the pathway appears to be controlled by a single regulator gene, co-ordinate repression and de-repression were not observed. Methionine also regulates its own synthesis by inhibition of the activity of homoserine -succinylase, the inhibitory effect being more marked with intact organisms than with cell-free extracts. Methionine did not influence the activity of cystathionine synthetase or the subsequent methionine-forming enzymes.


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