Molecular cloning of the hom-thrC-thrB cluster from Bacillus sp. ULM1: expression of the thrC gene in Escherichia coli and corynebacteria, and evolutionary relationships of the threonine genes.

A 6.5 kb DNA fragment containing the gene (thrC) encoding threonine synthase, the last enzyme of the threonine biosynthetic pathway, has been cloned from the DNA of Bacillus sp. ULM1 by complementation of Escherichia coli and Brevibacterium lactofermentum thrC auxotrophs. Complementation studies showed that the thrB gene (encoding homoserine kinase) is found downstream from the thrC gene, and analysis of nucleotide sequences indicated that the hom gene (encoding homoserine dehydrogenase) is located upstream of the thrC gene. The organization of this cluster of genes is similar to the Bacillus subtilis threonine operon (hom-thrC-thrB). An 1.9 kb BclI fragment from the Bacillus sp. ULM1 DNA insert 351 amino acids was found corresponding to a protein of 37462 Da. The thrC gene showed a low G + C content (39.4%) and the encoded threonine synthase is very similar to the B. subtilis enzyme. Expression of the 1.9 kb BcI DNA fragment in E. coli minicells resulted in the formation of a 37 kDa protein. The upstream region of this gene shows promoter activity in E. coli but not in corynebacteria. A peptide sequence, including a lysine that is known to bind the pyridoxal phosphate cofactor, is conserved in all threonine synthase sequences and also in the threonine and serine dehydratase genes. Amino acid comparison of nine threonine synthases revealed evolutionary relationships between different groups of bacteria.