Feedback Regulation of Lysine, Threonine, and Methionine Biosynthetic Enzymes in Corn¹

Abstract

The first three enzymes in the branched pathway for lysine, threonine, isoleucine, and methionine biosynthesis from aspartate were studied in five corn ( Zea mays L.) tissues. Extracts from 3‐day‐old etiolated shoots and roots of inbred W23, W23 kernels 22 days after pollination, and callus cultures and suspension cell cultures of ‘Black Mexican Sweet’ contained aspartokinase, aspartate semialdehyde dehydrogenase, and homoserine dehydrogenase activities. Low (0.02 to 0.1 m M ) lysine levels inhibited aspartokinase from each tissue and 0.025 to 0.25 m M threonlne inhibited homoserine dehydrogenase. Maximum threonine inhibition of homoserine dehydrogenase varied from 23 to 71% among tissues. Similar threonine‐sensitive homoserine dehydrogenase isozymes were found in crude extracts of each tissue. Combinations of lysine and threonine did not increase inhibition of either aspartokinase or homoserine dehydrogenase. Aspartate semialdehyde dehydrogenase of the callus and suspension culture extracts, but not the other extracts, was inhibited by 10 m M methionine; lysine and threonine were only slightly inhibitory alone or in combination. Conversion of aspartate to homoserine in vegetative and kernel tissues of corn thus is primarily feedback regulated by lysine at the first step, aspartokinase, and by threonine at the third step, homoserine dehydrogenase.