Cloning and transcriptional analysis of two threonine biosynthetic genes from Lactococcus lactis MG1614

Two genes, hom and thrB, involved in threonine biosynthesis in Lactococcus lactis MG1614, were cloned and sequenced. These genes, which encode homoserine dehydrogenase and homoserine kinase, were initially identified by the homology of their gene products with known homoserine dehydrogenases and homoserine kinases from other organisms. The identification was supported by construction of a mutant containing a deletion in hom and thrB that was unable to grow in a defined medium lacking threonine. Transcriptional analysis showed that the two genes were located in a bicistronic operon with the order 5' hom-thrB 3' and that transcription started 66 bp upstream of the translational start codon of the hom gene. A putative -10 promoter region (TATAAT) was located 6 bp upstream of the transcriptional start point, but no putative -35 region was identified. A DNA fragment covering 155 bp upstream of the hom translational start site was functional in pAK80, an L. lactis promoter probe vector. In addition, transcriptional studies showed no threonine-dependent regulation of hom-thrB transcription.