dehydratase, respectively; the pduGH genes encode proteins that are homologous to the DdrAB proteins, the two subunits of a reactivating factor for glycerol dehydratase in Klebsiella, the pduO gene has been shown to encode an adenosyltransferase, the pduLMSVX genes encode proteins with no convincing matches in the NCBI-nr database, and surprisingly seven genes, pduABJKNTU, encode products related to proteins required for the formation of the carboxysome, an organelle involved in CO2 fixation ((Bobik et al. 1999)).

Regulation

      The regulation of the pdu operon has been demonstrated to be quite complex (Figure 1-4). It is coregulated with the adjacent, divergently transcribed cobalamin synthetic operon (cob) and is controlled by both local and global regulatory systems. Locally, the positive regulatory protein, PocR, binds the P1, P2, Ppdu, and Pcob promoters when bound by its coeffector PD, inducing expression (Chen et al. 1995). Globally both the CRP/cAMP and the two-component ArcA/ArcB system, which sense carbon availability and environmental redox potential, respectively, exert control on the pdu/cob regulon (Escalante and Roth 1987, Andersson 1992, Ailion et al. 1993, Chen et al. 1995). The genes required for catabolism of PD and biosynthesis of the required cofactor, AdoB12, are co-induced both aerobically and anaerobically in S. enterica. Aerobic induction of these genes does not require the involvement of the ArcA/ArcB system but is solely reliant upon the CRP/cAMP system. Anaerobically the ArcA/ArcB system enhances expression of the pdu genes, and under these conditions the ArcA/ArcB and CRP/cAMP have an additive effect on expression. Studies have demonstrated that maximal induction of the pdu and cob operons occurs under anaerobic conditions during growth on poor carbon sources in the presence of PD (Bobik et al. 1992, Chen et al. 1995).