Vitamin B6 is an essential metabolite in all organisms. It can act as a coenzyme for numerous metabolic enzymes and has recently been shown to be a potent antioxidant. Plants and microorganisms have a de novo biosynthetic pathway for vitamin B6, but animals must obtain it from dietary sources. In Escherichia coli, it is known that the vitamin is derived from deoxyxylulose 5-phosphate (an intermediate in the nonmevalonate pathway of isoprenoid biosynthesis) and 4-phosphohydroxy-L-threonine. It has been assumed that vitamin B6 is synthesized in the same way in plants, but this hypothesis has never been experimentally proven. Here, we show that, in plants, synthesis of the vitamin takes an entirely different route, which does not involve deoxyxylulose 5-phosphate but instead utilizes intermediates from the pentose phosphate pathway, i.e., ribose 5-phosphate or ribulose 5-phosphate, and from glycolysis, i.e., dihydroxyacetone phosphate or glyceraldehyde 3-phosphate. The revelation is based on the recent discovery that, in bacteria and fungi, a novel pathway is in place that involves two genes (PDX1 and PDX2), neither of which is homologous to any of those involved in the previously doctrined E. coli pathway. We demonstrate that Arabidopsis thaliana has two functional homologs of PDX1 and a single homolog of PDX2. Furthermore, and contrary to what was inferred previously, we show that the pathway appears to be cytosolic and is not localized to the plastid. Last, we report that the single PDX2 homolog is essential for plant viability.Arabidopsis ͉ isoprenoid ͉ pyridoxine ͉ deoxyxylulose 5-phosphate V itamin B6 is well renowned in the medical field for being involved in more bodily functions than any other single nutrient. The vitamin exists in various forms, i.e., pyridoxal, pyridoxine, pyridoxamine, and their phosphorylated derivatives. As pyridoxal 5Ј-phosphate, it is an essential cofactor for numerous metabolic enzymes including amino acid metabolism and antibiotic biosynthesis. Most interestingly, it has recently been found that the vitamin is a potent antioxidant with a particular ability to quench reactive oxygen species such as superoxide and singlet oxygen (1, 2). The de novo biosynthesis of vitamin B6 takes place in microorganisms and plants, but this ability has been lost in animals, making it essential in the human diet.Despite the vitamin's obvious physiological and pharmaceutical importance, its biosynthesis has predominantly been studied in the Gram-negative bacterium, Escherichia coli, where it is derived from deoxyxylulose 5-phosphate (DXP, an intermediate in the nonmevalonate pathway of isoprenoid biosynthesis) and 4-phosphohydroxy-L-threonine (refs. 3-7 and Scheme 1). Astonishingly, although plants are a major source of vitamin B6 in the human diet, our understanding of the pathway therein is very limited and has been assumed to be derived the same way as in E. coli. There is one preliminary study that reports on its biosynthesis in spinach (8), whereas other, more in-depth studies have addressed the...

SummaryVitamin B6 is an essential coenzyme for numerous metabolic enzymes and is a potent antioxidant. In plants, very little is known about its contribution to viability, growth and development. The de novo pathway of vitamin B6 biosynthesis has only been described recently and involves the protein PDX1 (pyridoxal phosphate synthase protein). Arabidopsis thaliana has three homologs of PDX1, two of which, PDX1.1 and PDX1.3, have been demonstrated as functional in vitamin B6 biosynthesis in vitro and by yeast complementation. In this study, we show that the spatial and temporal expression patterns of PDX1.1 and PDX1.3, investigated at the transcript and protein level, largely overlap, but PDX1.3 is more abundant than PDX1.1. Development of single pdx1.1 and pdx1.3 mutants is partially affected, whereas disruption of both genes causes embryo lethality at the globular stage. Detailed examination of the single mutants, in addition to those that only have a single functional copy of either gene, indicates that although these genes are partially redundant in vitamin B6 synthesis, PDX1.3 is more requisite than PDX1.1. Developmental distinctions correlate with the vitamin B6 content. Furthermore, we provide evidence that in addition to being essential for plant growth and development, vitamin B6 also plays a role in stress tolerance and photoprotection of plants.

BackgroundNatural products are an important source of drugs and other commercially interesting compounds, however their isolation and production is often difficult. Metabolic engineering, mainly in bacteria and yeast, has sought to circumvent some of the associated problems but also this approach is impeded by technical limitations. Here we describe a novel strategy for production of diverse natural products, comprising the expression of an unprecedented large number of biosynthetic genes in a heterologous host.ResultsAs an example, genes from different sources, representing enzymes of a seven step flavonoid pathway, were individually cloned into yeast expression cassettes, which were then randomly combined on Yeast Artificial Chromosomes and used, in a single transformation of yeast, to create a variety of flavonoid producing pathways. Randomly picked clones were analysed, and approximately half of them showed production of the flavanone naringenin, and a third of them produced the flavonol kaempferol in various amounts. This reflected the assembly of 5–7 step multi-species pathways converting the yeast metabolites phenylalanine and/or tyrosine into flavonoids, normally only produced by plants. Other flavonoids were also produced that were either direct intermediates or derivatives thereof. Feeding natural and unnatural, halogenated precursors to these recombinant clones demonstrated the potential to further diversify the type of molecules that can be produced with this technology.ConclusionThe technology has many potential uses but is particularly suited for generating high numbers of structurally diverse compounds, some of which may not be amenable to chemical synthesis, thus greatly facilitating access to a huge chemical space in the search for new commercially interesting compounds

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