SNZ3 Encodes a PLP Synthase Involved in Thiamine Synthesis in Saccharomyces cerevisiae

Pyridoxal 5'-phosphate (the active form of vitamin B-6) 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.