The DapA gene encoding the lysine biosynthetic enzyme dihydrodipicolinate synthase from Coix lacryma-jobi:: cloning, characterization, and expression analysis

Dihydrodipicolinate synthase (DHPS) is the main enzyme of a specific branch of the aspartate pathway leading to lysine biosynthesis in higher plants. We have cloned and characterized the DHPS-encoding DapA gene from the maize-related grass Coix lacryma-jobi. The DapA open reading frame is interrupted by two introns and encodes the 326 amino acid-long Coix DHPS protein, which is 95% identical to the maize DHPS protein. Coix DNA gel blot analysis with maize DHPS cDNA as a probe showed a single strongly hybridizing band along with faint bands. RNA gel blot analysis showed that DHPS transcripts are present in coleoptiles, embryos, endosperms, and roots but are almost undetectable in blades of young leaves of both Coix and maize. The 5'-flanking region of the DapA gene contains a TGACTC GCN4-like element located 372 bp upstream the putative translation start codon. Steady-state levels of DHPS mRNA were slightly reduced in the endosperms and embryos of the maize lysine-rich opaque2 mutants when compared with those in normal kernels. Selective binding assay with the maize Opaque2 protein (O2) showed that the GCN4-like element is not an O2 binding site, suggesting that the DHPS gene is not under the control of O2.