Purine limitation prevents the exogenous pyridoxal 5'-phosphate accumulation of mutants.

Unlabelled: YggS belongs to the highly conserved pyridoxal 5'-phosphate (PLP) binding protein family (COG0325) that is found in all domains of life. Though no precise biochemical activity or molecular mechanism has been determined for this protein, an involvement in vitamin B homeostasis has been demonstrated in multiple organisms. In , loss of YggS results in altered B vitamer pools, including an accumulation of PLP in the growth medium. Transposon mutagenesis identified an insertion upstream of (encoding 5'-phosphoribosyl-5-aminoimidazole-4--succinocarboxamide synthetase, EC 6.3.2.6) that eliminated accumulation of PLP in the spent medium. Genetic characterization of the insertion showed the causative effect was reduced expression of , which limited purine biosynthesis. Data herein shows that purine limitation decreased the exogenous accumulation of B vitamers of a mutant but did not suppress other mutant phenotypes. Neither limitation for ATP, regulation by PurR, or decreased growth rate, all of which are direct consequences of purine limitation, prevented exogenous B vitamer accumulation of a mutant. This work establishes a relationship between the status of purine biosynthesis and the impact of a mutation. It lays the foundation for continued efforts to identify the physiological role of YggS and its homologs.

Importance: Pyridoxal 5'-phosphate is the active form of vitamin B and is an essential cofactor in all domains of life. PLP can be synthesized or salvaged from the environment from one of the six B vitamers. B vitamer levels appear to be tightly regulated, and alterations in their levels can have deleterious effects, most notably being the development of B6-dependent epilepsy in humans. YggS homologs are broadly conserved across multiple organisms and considered to be involved in maintaining B homeostasis, though no specific mechanism has been defined. The current study showed that the exogenous accumulation of PLP caused by a lack of YggS can be prevented by purine limitation. The demonstration that purine limitation impacts exogenous PLP accumulation separates one consequence of a mutation for further study and contributes to continuing efforts to define the biochemical and physiological roles of the COG0325 family of proteins.