CLONING AND EXPRESSION OF AN ARABIDOPSIS-THALIANA CDNA-ENCODING A MONOFUNCTIONAL ASPARTATE KINASE HOMOLOGOUS TO THE LYSINE-SENSITIVE ENZYMEOF ESCHERICHIA-COLI

As in many bacterial species, the first enzymatic reaction of the aspa rtate-family pathway in plants is mediated by several isozymes of aspa rtate kinase (AK) that are subject to feedback inhibition by the end-p roduct amino acids lysine or threonine. So far, only cDNAs and genes e ncoding threonine-sensitive AKs have been cloned from plants. These we re all shown to encode polypeptides containing two Linked activities, namely AK and homoserine dehydrogenase (HSD), similar to the Escherich ia coli thrA gene encoding a threonine-sensitive bifunctional AK/HSD i sozyme. In the present report, we describe the cloning of a new Arabid opsis thaliana cDNA that is relatively highly homologous to the E. col i lysC gene encoding the lysine-sensitive AK isozyme. Moreover, simila r to the bacterial lysine-sensitive AK, the polypeptide encoded by the present cDNA is monofunctional and does not contain an HSD domain. Th ese observations imply that our cloned cDNA encodes a lysine-sensitive AK. Southern blot hybridization detected a single gene highly homolog ous to the present cDNA, plus an additional much less homologous gene. This was confirmed by the independent cloning of an additional Arabid opsis cDNA encoding a lysine-sensitive AK (see accompanying paper). No rthern blot analysis suggested that the gene encoding this monofunctio nal AK cDNA is abundantly expressed in most if not all tissues of Arab idopsis.