Abstract
The regulation of homoserine dehydrogenase activity was studied in nineAzotobacter strains belonging to five different species. In all the species the enzyme is subject to feedback inhibition byl-threonine andl-isoleucine, the first being much more active as inhibitor. The inhibition byl-threonine is noncompetitive with respect to NADPH and of mixed type with respect to aspartate-Β-semialdehyde; the inhibition byl-isoleucine is noncompetitive with respect to both substrates. The synthesis of homoserine dehydrogenase inAzotobacter chroococcum I.P. is somewhat repressed by 1mm l-methionine and 5mm 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 speciesmacrocytogenes andinsignis than in thechroococcum, beijerinckii andvinelandii group (1.5–1.6). The pattern of control of this enzyme in the genusAzotobacter is discussed in relation to other bacterial homoserine dehydrogenases.
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