the methyl-citrate pathway (Witt et al. 1994, Horswill and Escalante-Semerena 1997, Textor et al. 1997, Walter et al. 1997, Tsang et al. 1998). In S. enterica the fermentation of PD provides an energy source but no source of carbon while aerobic respiration and anaerobic respiration using tetrathionate as an electron acceptor allow PD to be utilized as the sole source of carbon and energy in this organism (Price-Carter et al. 2001).

      In S. enterica, the genes required for the coenzyme B12-dependent catabolism of PD cluster at the PD utilization (pdu) locus. This locus consists of the positive transcription regulator, pocR, the PD diffusion facilitator, pduF, and the pdu operon (Figure 1-3). The regulation of the pdu operon has been investigated previously and studies have demonstrated that the pdu operon is controlled by the positive transcriptional regulator, PocR and the coeffector PD as well as the CRP/cAMP and the two component ArcA/ArcB global regulatory systems which sense carbon availability and oxygen levels in the cell environment, respectively (Bobik et al. 1992, Rondon and Escalante-Semerena 1992, Ailion et al. 1993, Rondon et al. 1995, Rondon and Escalante-Semerena 1997, Heithoff et al. 1999). Thepdu operon has been shown to be maximally induced under anaerobic conditions during growth on poor carbon sources in the presence of PD (Bobik et al. 1999).

      Recently our laboratory sequenced the pdu operon in its entirety and subsequent analysis suggested that it may encode a total of 21 genes; bringing the total number of pdu genes to 23 (Bobik et al. 1999). Six of the genes are thought to encode enzymes needed for the PD degradative pathway; two are involved in transport and regulation; two are probably involved in diol dehydratase reactivation; one is needed for the conversion