Recent Progress of Vitamin B_6 Biosynthesis

This review is the current summary of vitamin B_6 (B_6) biosynthesis in Escherichia coli and other microorganisms. The de novo biosynthesis of B_6 has been studied extensively for last three decades. However, the de novo biosynthesis of B_6 still remains unclear in spite of its simple structure. For the first two decades, B_6 biosynthesis had been mainly studied with E. coli using genetic, nutritional, and isotopic labeling experiments. According to these studies, some compounds including glycolaldehyde were identified as the precursor. During the last decade, gene manipulate techniques were rapidly developed, and complete genome sequences of some microorganisms became available. Using these new tools, valuable information has been provided. The complete DNA sequence of pdx genes and other genes, which are possibly involved in B_6 biosynthesis, were shown. The roles of some genes and precursors were proposed. Besides E. coli, B_6 biosynthesis in other microorganisms has been also studied. In some microorganisms, snz/sno was reported to be involved in B_6 biosynthesis. Intriguingly, these genes show no similarity to any of the E. coli pdx genes, and are not found in E. coli. Microorganisms having snz/sno gene homologues lack homologues to pdxA/pdxJ genes, whereas those with homologues to pdxA/pdxJ lack snz/sno gene homologues. Therefore, it is most likely that there are at least two kinds of B_6 biosynthetic pathways in microorganisms. These studies provided important clues of B_6 biosynthesis, but the entire picture of the B_6 biosynthetic pathway remains unclear.