however, that are components of the active coenzyme forms: adenosylcobalamin (AdoB12) and methylcobalamin (CH3-B 12).

                                Biosynthesis of B12

      In S. enterica the genes required for the biosynthesis of B12 or cobalamin are located at the cob operon. This operon is coregulated with genes required for the degradation of propanediol (PD), which are located in the adjacent and divergently transcribed pdu operon, indicating that degradation of PD may be the primary reason for de novo synthesis of B12 in S. enterica. Several pathways for B12 synthesis exist in microorganisms, some occur aerobically, and others can occur only in the absence of oxygen. In Klebsiella aerogenes, B12 is made both aerobically and anaerobically, making it an ideal organism to study the differences in these pathways. In S. enterica however, B12 is made only under anaerobic conditions.

                                 Transport of B12

      In addition to synthesizing B12 de novo, S. enterica can scavenge corrinoids from its environment for conversion into B12. The transport of B12 in Salmonella is rather problematic since B12 is present only in minute quantities in the environment and is also a large molecule that exceeds the limit for entry via outer membrane porins. The mechanism of B12 transport has been extensively studied in E. co/i and consists of two separate transport systems, one for the outer membrane and a second for the inner membrane (Bradbeer 1991). Transport across the outer membrane entails binding to the BtuB protein which has a high affinity for vitamin B12 and its derivatives (Taylor et al. 1972, White et al. 1973, Bradbeer et al. 1978, Kenley et al. 1978). Once bound to BtuB, the B12-BtuB complex interacts with the TonB protein which provides energy derived from a proton-motive force to drive a structural alteration of the BtuB protein resulting in