Molecular Genetics of the Maize ( Zea mays 1 . ) Aspartate Kinase - H omoserine Dehydrogenase Gene Family ’ Gary

Aspartate kinase (AK) and homoserine dehydrogenase (HSDH) are enzymes in the aspartate-derived amino acid biosynthetic pathway. Recent biochemical evidence indicates that an AK-HSDH bifunctional enzyme exists in maize (Zea mays 1.). In this report, we characterize three genes that encode subunits of AK-HSDH. Two cDNAs, pAKHSDHl and pAKHSDH2, containing the fullcoding sequence, and one partial cDNA, pAKHSDH3, encode amino acid sequences similar to the reported monofunctional AK and HSDH enzymes from prokaryotes and yeast (Saccharomyces cerevisiae) and to AK-HSDH bifunctional enzymes of prokaryotes, yeast, carrot (Daucus carota), and Arabidopsis thaliana. Immunological and biochemical analyses verify that the cDNAs encode AKHSDH and indicate that both the AK and HSDH activities are feedback inhibited by threonine. RNA blots identify a 3.2-kb transcript in all maize tissues examined. pAKHSDHl and pAKHSDH2 map to chromosomes 41 and 2S, respectively. This study shows that maize contains AK-HSDH bifunctional enzyme(s) encoded by a small gene family of at least three genes. Maize AK-HSDH has conserved sequences found in communication modules of prokaryotic two-component regulatory systems, which has led us to propose that maize AK-HSDH may be involved in a similar regulatory mechanism.