Tg. Fazzio et Jr. Roth, EVIDENCE THAT THE CYSG PROTEIN CATALYZES THE FIRST REACTION SPECIFIC TO B-12 SYNTHESIS IN SALMONELLA-TYPHIMURIUM, INSERTION OF COBALT, Journal of bacteriology, 178(23), 1996, pp. 6952-6959
The cysG gene of Salmonella typhimurium is involved in synthesis of bo
th cobalamin (B-12) and siroheme (a cofactor required for SO32- and NO
22- reductases). The failure to reduce SO32- leads to cysteine auxotro
phy, for which the enzyme is named, Although Escherichia coli does not
synthesize B-12 de novo, it possesses a very similar CysG enzyme whic
h has been shown to catalyze two methylations (uroporphyrinogen III to
precorrin-2), ring oxidation (precorrin-2 to factor II), and iron ins
ertion (factor II to siroheme). In S. typhimurium, precorrin-2 is a pr
ecursor of both siroheme and B-12. All previously known Salmonella cys
G mutants are defective in the synthesis of both siroheme and cobalami
n. We describe two new classes of cysG mutants that cannot synthesize
B-12 but still make siroheme. For class I mutants, exogenous cobalt co
rrects the B-12 defect but inhibits ability to make siroheme; B-12 syn
thesis is inhibited by added iron, Class II mutants are unaffected by
exogenous cobalt, but their B-12 defect is corrected by derepression o
f the B-12 biosynthetic genes (cob). We propose that all mutants are d
efective in insertion of cobalt into factor II and that the Salmonella
CysG enzyme normally catalyzes this insertion-the first reaction dedi
cated to cobalamin synthesis. Although E. coli does not make B its Cys
G enzyme has been shown in vitro to insert cobalt into factor II and m
ay have evolved to support B-12 synthesis in some ancestor common to S
almonella species and E. coli.