MOLECULAR-GENETICS OF THE MAIZE (ZEA-MAYS L) ASPARTATE KINASE HOMOSERINE DEHYDROGENASE GENE FAMILY

Aspartate kinase (AK) and homoserine dehydrogenase (HSDH) are enzymes in the aspartate-derived amino acid biosynthetic pathway. Recent bioch emical evidence indicates that an AK-HSDH bifunctional enzyme exists i n maize (Zea mays L.). In this report, we characterize three genes tha t encode subunits of AK-HSDH. Two cDNAs, pAKHSDH1 and pAKHSDH2, contai ning the full-coding sequence, and one partial cDNA, pAKHSDH3, encode amino acid sequences similar to the reported monofunctional AK and HSD H enzymes from prokaryotes and yeast (Saccharomyces cerevisiae) and to AK-HSDH bifunctional enzymes of prokaryotes, yeast, carrot (Daucus ca rota), and Arabidopsis thaliana. Immunological and biochemical analyse s verify that the cDNAs encode AKHSDH and indicate that both the AK an d HSDH activities are feedback inhibited by threonine. RNA blots ident ify a 3.2-kb transcript in all maize tissues examined. pAKHSDH1 and pA KHSDH2 map to chromosomes 4L and 2S, respectively. This study shows th at maize contains AK-HSDH bifunctional enzyme(s) encoded by a small ge ne family of at least three genes. Maize AK-HSDH has conserved sequenc es found in communication modules of prokaryotic two-component regulat ory systems, which has led us to propose that maize AK-HSDH may be inv olved in a similar regulatory mechanism.