Siroheme, the cofactor for
sulfite and
nitrite reductases, is formed by methylation, oxidation, and
iron insertion into the
tetrapyrrole uroporphyrinogen III (Uro-III). The CysG
protein performs all three steps of
siroheme biosynthesis in the enteric bacteria Escherichia coli and Salmonella enterica. In either taxon, cysG mutants cannot reduce
sulfite to
sulfide and require a source of
sulfide or
cysteine for growth. In addition, CysG-mediated methylation of Uro-III is required for de novo synthesis of
cobalamin (
coenzyme B(12)) in S. enterica. We have determined that cysG mutants of the related enteric bacterium Klebsiella aerogenes have no defect in the reduction of
sulfite to
sulfide. These data suggest that an alternative
enzyme allows for
siroheme biosynthesis in CysG-deficient strains of Klebsiella. However, Klebsiella cysG mutants fail to synthesize
coenzyme B(12), suggesting that the alternative
siroheme biosynthetic pathway proceeds by a different route. Gene cysF, encoding an alternative
siroheme synthase homologous to CysG, has been identified by genetic analysis and lies within the cysFDNC operon; the cysF gene is absent from the E. coli and S. enterica genomes. While CysG is coregulated with the
siroheme-dependent
nitrite reductase, the cysF gene is regulated by
sulfur starvation. Models for alternative regulation of the CysF and CysG
siroheme synthases in Klebsiella and for the loss of the cysF gene from the ancestor of E. coli and S. enterica are presented.