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 me thylotroph Methylobacillus flagellatus were cloned. In maxicells hom and th rC directed synthesis of 51 and 48 kDa polypeptides, respectively. The hem, thrB and thrC genes and adjacent DNA areas were sequenced. Of the threonin e biosynthesis genes, only horn and thrC were tightly linked in the order h om-thrC The gene for thymidylate synthase (thyA) followed thrC and the gene for aspartate aminotransferase (aspC) preceded hem. All four genes (aspC-h om-thrC-thyA) were transcribed in the same direction. mRNA analysis indicat ed 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 hem. No functional promoter was found to be associated wi th the DNA stretch between hom and thrC, The thrB gene encoded an unusual t ype of homoserine kinase and was not linked to other threonine biosynthesis genes.