PLANT PHYSIOLOGY , Vol 106, Issue 4 1303-1312, Copyright © 1994 by American Society of Plant Biologists
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MOLECULAR BIOLOGY AND GENE REGULATION |
Molecular Genetics of the Maize (Zea mays L.) Aspartate Kinase-Homoserine Dehydrogenase Gene Family
G. J. Muehlbauer, D. A. Somers, B. F. Matthews and B. G. Gengenbach
Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, Minnesota 55108 (G.J.M., D.A.S., B.G.G.)
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 L.). In this report, we characterize three genes that encode subunits
of AK-HSDH. Two cDNAs, pAKHSDH1 and pAKHSDH2, containing the full-coding
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.
Immuno-logical and biochemical analyses verify that the cDNAs encode
AK-HSDH 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. pAKHSDH1 and pAKHSDH2 map to chromosomes 4L 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.
