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Pyridoxal 5′-phosphate (the active form of vitamin B6) is a cofactor that is important for a broad number of biochemical reactions and is essential for all forms of life. Organisms that can synthesize pyridoxal 5′-phosphate use either the deoxyxylulose phosphate-dependent or -independent pathway, the latter is encoded by a two-component pyridoxal 5′-phosphate synthase. Saccharomyces cerevisiae contains three paralogs of the two-component SNZ/SNO pyridoxal 5′-phosphate synthase. Past work identified the biochemical activity of Snz1p, Sno1p and provided in vivo data that SNZ1 was involved in pyridoxal 5′-phosphate biosynthesis. Snz2p and Snz3p were considered redundant isozymes and no growth condition requiring their activity was reported. Genetic data herein showed that either SNZ2 or SNZ3 are required for efficient thiamine biosynthesis in Saccharomyces cerevisiae. Further, SNZ2 or SNZ3 alone could satisfy the cellular requirement for pyridoxal 5′-phosphate (and thiamine), while SNZ1 was sufficient for pyridoxal 5′-phosphate synthesis only if thiamine was provided. qRT-PCR analysis determined that SNZ2,3 are repressed ten-fold by the presence thiamine. In total, the data were consistent with a requirement for PLP in thiamine synthesis, perhaps in the Thi5p enzyme, that could only be satisfied by SNZ2 or SNZ3. Additional data showed that Snz3p is a pyridoxal 5′-phosphate synthase in vitro and is sufficient to satisfy the pyridoxal 5′-phosphate requirement in Salmonella enterica when the medium has excess ammonia.

Pyridoxal 5'-phosphate (the active form of vitamin B6) is a cofactor that is important for a broad number of biochemical reactions and is essential for all forms of life. Organisms that can synthesize pyridoxal 5'-phosphate use either the deoxyxylulose phosphate-dependent or -independent pathway, the latter is encoded by a two-component pyridoxal 5'-phosphate synthase. Saccharomyces cerevisiae contains three paralogs of the two-component SNZ/SNO pyridoxal 5'-phosphate synthase. Past work identified the biochemical activity of Snz1p, Sno1p and provided in vivo data that SNZ1 was involved in pyridoxal 5'-phosphate biosynthesis. Snz2p and Snz3p were considered redundant isozymes and no growth condition requiring their activity was reported. Genetic data herein showed that either SNZ2 or SNZ3 are required for efficient thiamine biosynthesis in Saccharomyces cerevisiae. Further, SNZ2 or SNZ3 alone could satisfy the cellular requirement for pyridoxal 5'-phosphate (and thiamine), while SNZ1 was sufficient for pyridoxal 5'-phosphate synthesis only if thiamine was provided. qRT-PCR analysis determined that SNZ2,3 are repressed ten-fold by the presence thiamine. In total, the data were consistent with a requirement for PLP in thiamine synthesis, perhaps in the Thi5p enzyme, that could only be satisfied by SNZ2 or SNZ3. Additional data showed that Snz3p is a pyridoxal 5'-phosphate synthase in vitro and is sufficient to satisfy the pyridoxal 5'-phosphate requirement in Salmonella enterica when the medium has excess ammonia. [ABSTRACT FROM AUTHOR]

Abstract: Vitamin B6, an essential cofactor in enzymatic reactions, has only recently been linked to cellular oxidative stress. We investigated the role of this vitamin as an antioxidant in oxidative responses linked to plant defense. B6 vitamers effectively quenched superoxide and had antioxidant activity when assayed in vitro. The de novo B6 biosynthetic genes (PDX1 and PDX2) were identified in Nicotiana tabacum cv. ‘Burley 21’ and their transcript abundance was assayed during defense responses. PDX1 and PDX2 transcript levels decreased following inoculation with the incompatible pathogen Pseudomonas syringae pv. phaseolicola and transiently increased in response to salicylic acid and methyl jasmonate. Excess vitamin B6 in tobacco leaves interfered with the development of a hypersensitive response caused by P. syringae pv. phaseolicola and increased disease severity caused by the compatible bacterium P. syringae pv. tabaci. Our findings indicate that during plant defense responses, vitamin B6 functions and its synthesis is regulated in a manner consistent with this vitamin''s activity as an antioxidant and modulator of active oxygen species in vivo. [Copyright &y& Elsevier]