Functional Genomic, Biochemical, and Genetic Characterization of the Salmonella pduO Gene, an ATP:Cob(I)alamin Adenosyltransferase Gene

Salmonella enterica catabolizes 1,2-propanediol via a pathway that is dependent upon adenosyl cobalamin (AdoCbl), a metabolically active form of vitamin B12 (18). Since 1,2-propanediol is formed by the fermentation of the common plant sugars rhamnose and fucose, its catabolism may provide a selective advantage in anaerobic environments (20, 25). Studies employing in vivo expression technology and competitive index assays have suggested that 1,2-propanediol degradation may also provide a growth advantage in host tissues (10, 15). A number of these aspects of Salmonella biology have been reviewed recently (27). The genes for 1,2-propanediol utilization (pdu) are found at centisome 44 of the S. enterica genetic map (18). They are adjacent to and coregulated with 20 genes for de novo AdoCbl synthesis (1, 5, 26, 28). These genes are absent from Escherichia coli, and evidence indicates they were acquired by S. enterica via a single horizontal gene transfer (6, 28). Surprisingly, there are 23 pdu genes (6). Six of these probably encode enzymes of the degradative pathway, and four are thought to be involved in regulation, transport, and diol dehydratase reactivation (5, 6, 9). Perhaps five to seven pdu genes are needed for formation of a polyhedral body that is associated with the diol dehydratase, and the remaining six are of unknown function (6). The proposed pathway of 1,2-propanediol degradation begins with its conversion to propionaldehyde by AdoCbl-dependent diol dehydratase (25, 33). Next, the aldehyde is disproportionated to either propanol or propionic acid. Alcohol dehydrogenase, aldehyde dehydrogenase, phosphotransacylase, and propionate kinase are thought to catalyze this process. The pathway yields ATP, an electron sink, and an intermediate (propionyl-coenzyme A [CoA]), which can feed into central metabolism via the methyl-citrate pathway (16). The AdoCbl needed for 1,2-propanediol degradation can be obtained either by de novo synthesis or by the assimilation of an exogenous cobalamin (Cbl). In S. enterica, de novo synthesis occurs only under strictly anaerobic conditions (19). However, Cbls, such as cyanocobalamin (vitamin B12 [CNCb]) and hydroxycobalamin (HOCbl), can be assimilated both aerobically and anaerobically (18). The conversion of CNCbl and HOCbl to AdoCbl is thought to proceed as shown in Fig. ​Fig.1:1: CNCbl is decyanated to HOCbl, reduced to cob(II)alamin, further reduced to cob(I)alamin, and finally adenosylated to AdoCbl (14, 17). The cobA gene (which maps far from the pdu/cob locus at centisome 34) encodes an ATP:corrinoid adenosyltransferase (32). This enzyme participates in Cbl assimilation by adenosylation of cob(I)alamin and also in de novo AdoCbl synthesis by the adenosylation of an intermediate prior to Cbl (13). Genetic tests have indicated that Cbl assimilation genes are also found in the pdu operon, but the specific genes involved have not been identified (37; T. A. Bobik, unpublished results). FIG. 1 Proposed vitamin B12 adenosylation pathway. Coenzyme B12 (a required cofactor for a number of enzymes) is thought to be produced from vitamin B12 by the series of reactions shown. The corrin ring of vitamin B12 is shown lightly shaded, the nucleotide ... Here we report that the pduO gene encodes an ATP:cob(I)alamin adenosyltransferase used in Cbl assimilation for 1,2-propanediol degradation by S. enterica. We also present the results of a functional genomic analysis of adenosyltransferases that indicates the following: (i) particular adenosyltransferases tend to be specialized for particular AdoCbl-dependent enzymes, (ii) the PduO adenosyltransferase is a bifunctional enzyme, and (iii) unknown genes proximal to adenosyltransferase homologues may represent previously unidentified AdoCbl-dependent enzymes.