Pyridine transport and assimilation in salmonella enterica

The goal of this work was to reveal how various pyridines are transported and assimilated in Salmonella enterica. In the first strategy, mutants were isolated that allow the use of nicotinamide mononucleotide (NMN) as a sole pyridine source when all known import routes are blocked. The major responsible mutations (pnuP*) affected a transmembrane-spanning protein, whose closest homolog is YegT, a putative nucleoside transporter. A minor class (pnuP*) affected the ydeA gene encoding a homolog of drug:H+ antiporters. It is suggested that PnuD and PnuP are both adapted transporters that have been altered to transport nicotinamide ribonucleoside (NmR). As seen for the PnuC transporter, assimilation of NMN using PnuD* or PnuP* depended on AphA and Nadl(T) functions. It is suggested that NMN is converted to NmR prior to import through PnuC, PnuD*, or PnuP*, and NmR is phosphorylated by Nadl(T) after entry. In the second strategy, mutants were isolated that improve quinolinic acid (Qa) assimilation. Mutants able to grow on a low concentration of Qa affected eight different loci. The affected genes were grouped into three categories based on their mode of action. Group I included four classes of mutations that recruited an adapted transporter: the pnuF* (yabN), pnuG* (opp), pnuH* (kefA), and pnul* mutations. It is suggested that wild-type PnuF and Opp may contribute to pyridine transport under some conditions. Group II comprised mutations in genes encoding enzymes involved in glycolysis: pgi, pfkA, and ptsG. It is proposed that these mutations enhance Qa assimilation by increasing the level of cAMP, which induces Qa transporter(s). Group III (pnuM) appeared to amplify the gabP gene encoding the GABA permease. This is the first report, in which the GABA transporter has been shown to transport Qa in any organism.