Concerted Feedback Inhibition of the Aspartokinase ofRhodospirillum tenueby Threonine and Methionine: a Novel Pattern

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Abstract
The presence of a single aspartokinase was demonstrated inRhodospirillum tenue. The enzyme has been purified about 60-fold. No physical association exists in this species between aspartokinase and homoserine dehydrogenase. The general properties of the enzyme are described. Inhibition byl-lysine, byl-threonine, and concerted inhibition by these two end products are regulatory characters which have also been found in many other species. InR. tenue, aspartokinase is also subject to a hitherto not encountered type of concerted feedback inhibition, byl-threonine plusl-methionine. The inhibition caused by lysine can be reversed either by glycine,l-isoleucine,l-methionine, orl-phenylalanine. The concerted inhibition by lysine plus threonine is reversed by glycine,l-isoleucine, orl-phenylalanine, but not byl-methionine, which exerts in conjunction with threonine the independent concerted inhibition referred to above. Addition of single or several metabolites to cultures ofR. tenuecaused inhibition of growth and reversal of growth inhibition, compatible with the effects observed in vitro on aspartokinase activity. The regulation of this enzyme in relation to that of other bacterial aspartokinases is discussed.