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SALA-TREPAT, J. M., ROBERT-GERo, M. and LE BORGNE, L. 1972. Regulation of homoserine dehydrogenase in Azotobacter species. Antonie van Leeuwenhoek: 38: 493-503.The regulation of homoserine dehydrogenase activity was studied in nine Azotobacter strains belonging to five different species. In all the species the enzyme is subject to feedback inhibition by L-threonine and L-isoleucine, the first being much more active as inhibitor. The inhibition by L-threonine is noncompetitive with respect to NADPH and of mixed type with respect to aspartate-f3-semialdehyde; the inhibition by L-isoleucine is noncompetitive with respect to both substrates. The synthesis of homoserine dehydrogenase in Azotobacter chroococcum I.P. is somewhat repressed by 1 mM L-methionine and 5 mM L-isoleucine. In all the strains examined either NADPH or NADH can serve as cofactors for this activity, though the ratio of activity with the two pyridine nucleotides (NADPH/NADH) shows higher values (3.3-3.8) in the species macrocytogenes and insignis than in the chroococcum, beijerhTckii and vinelandii group (1.5-1.6). The pattern of control of this enzyme in the genus Azotobacter is discussed in relation to other bacterial homoserine dehydrogenases.

The presence of a single aspartokinase was demonstrated in Rhodospirillum 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 by l -lysine, by l -threonine, and concerted inhibition by these two end products are regulatory characters which have also been found in many other species. In R. tenue , aspartokinase is also subject to a hitherto not encountered type of concerted feedback inhibition, by l -threonine plus l -methionine. The inhibition caused by lysine can be reversed either by glycine, l -isoleucine, l -methionine, or l -phenylalanine. The concerted inhibition by lysine plus threonine is reversed by glycine, l -isoleucine, or l -phenylalanine, but not by l -methionine, which exerts in conjunction with threonine the independent concerted inhibition referred to above. Addition of single or several metabolites to cultures of R. tenue caused 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.

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