S. enterica, the degradation of PD occurs aerobically, or anaerobically when tetrathionate is supplied as an electron acceptor (Price-Carter et al. 2001).

      The complexity of PD degradation became apparent when the DNA sequence of the pdu locus was determined, and twenty-threepdu genes were identified: six pdu 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 of vitamin B12 (CN-B12) to coenzyme B12; five are of unknown function; and seven share similarity to genes involved carboxysome formation (Bobik et al. 1999). Carboxysomes are polyhedral organelles found in cyanobacteria and some chemoautotrophs (Shively et al. 1973b, Shively and English 1991, Shively et al. 1998). They are composed of a proteinaceous shell that houses most of the cell's ribulose bisphosphate carboxylase/oxygenase (RuBisCO). They are required for autotrophic growth at low CO2 concentrations and are thought to function as part of a CO2concentrating mechanism (Price and Badger 1991, Price et al. 1998, Kaplan and Reinhold 1999).

      Recently, S. enterica was shown to form polyhedral organelles that resemble

carboxysomes during aerobic and anaerobic growth on PD (Bobik et al. 1999). These organelles are approximately 150 nm in cross-section and appear to consist of a proteinaceous shell and interior (Bobik et al. 1999). However, there are significant differences between the S. enterica organelles and carboxysomes. S. enterica is not an autotroph and does not express RuBisCO. The S enterica organelles are involved in coenzyme B12-dependent PD degradation and are associated with coenzyme B12dependent diol dehydratase and perhaps other enzymes (Bobik et al. 1999). A role in