Organization of threonine biosynthesis genes from the obligate methylotroph Methylobacillus flagellatus

The genes encoding aspartate kinase (ask), homoserine dehydrogenase (hom), homoserine kinase (thrB) and threonine synthase (thrC) from the obligate methylotroph Methylobacillus flagellatus were cloned. In maxicells hom and thrC directed synthesis of 51 and 48 kDa polypeptides, respectively. The hom, thrB and thrC genes and adjacent DNA areas were sequenced. Of the threonine biosynthesis genes, only hom and thrC were tightly linked in the order hom-thrC. The gene for thymidylate synthase (thyA) followed thrC and the gene for aspartate aminotransferase (aspC) preceded hom. All four genes (aspC-hom-thrC-thyA) were transcribed in the same direction. mRNA analysis indicated that hom-thrC are apparently transcribed in one 7.5 kb transcript in M. flagellatus. Promoter analysis showed the presence of a functional promoter between aspC and hom. No functional promoter was found to be associated with the DNA stretch between hom and thrC. The thrB gene encoded an unusual type of homoserine kinase and was not linked to other threonine biosynthesis genes.