To better understand the physiology and molecular biology of AdoCbl-dependent processes, 1,2-propanediol degradation by S. enterica
serovar Typhimurium LT2 was investigated. The DNA sequence of the pdu
operon was completed and analyzed, and evidence was presented that polyhedral organelles are involved in AdoCbl-dependent 1,2 propanediol degradation. Analyses of previously unpublished pdu
DNA sequence substantiated previous studies indicating that the pdu
operons were acquired by a single horizontal gene transfer event (30
) and allowed the identification of 16 hypothetical genes. In all, 23 pdu
genes are proposed, and these genes fall into six classes: pathway, diol dehydratase reactivation, unknown, polyhedral body formation, transport, and regulation (Fig. and Table ).
FIG. 3 The pdu locus of S. enterica with a summary of the encoded functions. Genes thought to be involved in the formation of polyhedra are indicated above the genetic map. Genes thought to be involved in the pathway of 1,2-propanediol degradation and in the (more ...)
With one exception, pdu genes corresponding to each enzyme of the proposed 1,2-propanediol degradative pathway have now been identified (Fig. ). A pdu gene corresponding to a proposed phosphotransacylase was not identified. This enzyme might be encoded by one of the six pdu genes of unknown function, or perhaps the proposed CoA-dependent aldehyde dehydrogenase (PduP) is bifunctional or a kinase; a Blast-ProDom search showed that the PduP protein shares a domain with ProA proteins, enzymes that catalyze the reduction of glutamate-5-semialdehyde to gamma-glutamyl-5-phosphate.
Six hypothetical Pdu proteins of unknown function (PduLMOSVX) were also identified. Genetic tests have shown that the pdu
operon encodes functions for the conversion of vitamin B12
(CN-Cbl) to AdoCbl, the active cofactor of diol dehydratase. The PduG protein has been implicated in this process, but additional Pdu proteins might also be involved. Studies with several different systems have indicated that a decyanase, one or two cobalt reductases, and an adenosyltransferase are needed (22
). The PduL protein is a possibility, since related proteins are found in an operon that encodes an AdoCbl-dependent glycerol dehydratase (17
). Other possible functions for Pdu proteins are suggested by physiological studies. S. enterica
grows via the anaerobic respiration of 1,2-propanediol with tetrathionate as a terminal electron acceptor (9
). Hence, some Pdu proteins might be specific to the 1,2-propanediol–tetrathionate respiration. The PduS protein is a possibility, since it is related in amino acid sequence to several membrane-bound oxidoreductases. In addition, previous studies have indicated that the pdu
operon encodes a protein involved in regulation of the prpBCDE
). Hence, one of the Pdu proteins of unknown function may fulfill this regulatory role.
The polyhedral bodies formed by S. enterica
during growth on 1,2-propanediol were also investigated. Based on results reported here and previous results, we propose that S. enterica
forms polyhedral organelles involved in 1,2-propanediol degradation that consist of AdoCbl-dependent diol dehydratase (and perhaps other proteins) encased within a protein shell related to the shell of carboxysomes. During growth on 1,2-propanediol, S. enterica
forms polyhedra that are proteinaceous in nature and that have sharp edges indicative of a shell (this study and reference 46
). Immunogold labeling indicated that diol dehydratase is associated with the polyhedra and that it is localized to the interior of these structures. DNA sequence analyses showed that the pdu
operon encodes five to seven proteins that are related to those involved in the formation of carboxysomes (this study and reference 13
), and genetic tests showed that genes of the pdu
operon are required for polyhedral body formation. In addition, a pdu
mutant was found that produced aberrantly shaped polyhedra. Thus, some pdu
genes are required for proper organelle shape, while others apparently encode its basic structural components. Analogous Synechococcus
mutants (that produce aberrantly shaped carboxysomes) were previously identified (35
Although the polyhedral bodies involved in 1,2-propanediol degradation are apparently related to carboxysomes structurally, a functional relationship is uncertain. Carboxysomes are proposed to play a role in concentrating CO2
for RuBisCo, since mutations in shell genes result in strains that require high CO2
for autotrophic growth (36
). On the other hand, the polyhedra of S. enterica
function in AdoCbl-dependent catabolism of 1,2-propanediol, and this process has no known association with CO2
. Previous reports, which identified the carboxysome shell protein gene homologues in the eut
operons of S. enterica
, discussed some possible functions for polyhedral bodies in the AdoCbl-dependent catabolism of ethanolamine and 1,2-propanediol (13
). It was suggested that polyhedral bodies could be used to sequester toxic aldehydes formed both during 1,2-propanediol and ethanolamine degradation and channel them to subsequent pathway enzymes. It was also suggested that polyhedra might be used to protect diol dehydratase and ethanolamine ammonia-lyase from oxygen, a molecule to which both are sensitive (13
). The finding reported here, that AdoCbl-dependent diol dehydratase is associated with polyhedra, is consistent with both of these hypotheses.
Although their precise function is unknown, the size of the polyhedral organelles and the number of genes involved attest to the substantial resources devoted to AdoCbl-dependent 1,2-propanediol degradation. Formation of these bodies must play an important role in S. enterica survival and niche establishment among the competitive flora of natural environments.