The alternative electron acceptor tetrathionate supports B-12-dependent anaerobic growth of Salmonella enterica serovar typhimurium on ethanolamine or 1,2-propanediol

Synthesis of cobalamin de novo by Salmonella enterica serovar Typhimurium s train LT2 and the absence of this ability in Escherichia coli present sever al problems. This large synthetic pathway is shared by virtually all salmon ellae and must be maintained by selection, yet no conditions are known unde r which growth depends on endogenous B-12, The cofactor is required for deg radation of 1,2-propanediol and ethanolamine. However, cofactor synthesis o ccurs only anaerobically, and neither of these carbon sources supports anae robic growth with any of the alternative electron accepters tested thus far . This paradox is resolved by the electron acceptor tetrathionate, which al lows Salmonella to grow anaerobically on ethanolamine or 1,2-propanediol by using endogenously synthesized B-12. Tetrathionate provides the only known conditions under which simple cob mutants (unable to make B-12) show a gro wth defect. Genes involved in this metabolism include the ttr operon, which encodes tetrathionate reductase. This operon is globally regulated by OxrA (Fnr) and induced anaerobically by a two-component system in response to t etrathionate. Salmonella reduces tetrathionate to thiosulfate, which it can further reduce to H2S, by using enzymes encoded by the genes phs and asr, The genes for 1,2-propanediol degradation (pdu) and B-12 synthesis (cob), a long with the genes for sulfur reduction (ttr, phs, and asr), constitute mo re than 1% of the Salmonella genome and are all absent from E. coli, In div erging from E. coli, Salmonella acquired some of these genes unilaterally a nd maintained others that are ancestral but have been lost from the E. coli lineage.