The Role of YggS in Vitamin B6 Homeostasis in Salmonella enterica Is Informed by Heterologous Expression of Yeast SNZ3

YggS (COG0325) is a pyridoxal 5'-phosphate (PLP)-binding protein proposed to be involved in homeostasis of B-6 vitamers. In Salmonella enterica, lack of yggS resulted in phenotypes that were distinct and others that were similar to those of a yggS mutant of Escherichia coll. Like other organisms, yggS mutants of S. enterica accumulate endogenous pyridoxine 5'-phosphate (PNP). Data herein show that strains lacking YggS accumulated -10-fold more PLP in growth medium than a parental strain. The deoxyxylulose 5-phosphate-dependent biosynthetic pathway for PLP and the PNP/pyridoxamine 5'-phosphate (PMP) oxidase credited with interconverting B-6 vitamers were replaced with a single PLP synthase from Saccharomyces cerevisiae. The impact of a yggS deletion on the intracellular and extracellular levels of B-6 vitamers in this restructured strain supported a role for PdxH in PLP homeostasis and led to a general model for YggS function in PLP-PMP cycling. Our findings uncovered broader consequences of a yggS mutation than previously reported and suggest that the accumulation of PNP is not a direct effect of lacking YggS but rather a downstream consequence. IMPORTANCE Pyridoxal 5'-phosphate (PLP) is an essential cofactor for enzymes in all domains of life. Perturbations in PLP or B-6 vitamer content can be detrimental, notably causing B-6-dependent epilepsy in humans. YggS homologs are broadly conserved and have been implicated in altered levels of B-6 vitamers in multiple organisms. The biochemical activity of YggS, expected to be conserved across domains, is not yet known. Herein, a simplified heterologous pathway minimized metabolic variables and allowed the dissection of this system to generate new metabolic knowledge that will be relevant to understanding YggS.