only minor components of their respective polyhedral organelles. Five additional pdu proteins that were undetectable in purified organelles (PduLMSVX) are of unknown function and lack convincing homology to proteins of known function currently in the NCBI nr database. These proteins might be needed for organelle assembly, but are absent from mature structures. Alternatively, they might encode additional enzymes needed for PD degradation or B12 metabolism that function outside the polyhedral organelles.

      Overall, the analyses presented in this report indicate the pdu organelles consist of a shell composed of 4-7 proteins that encase 4 enzymes. The shell of thepdu organelles appears to be related to the shell of carboxysomes since 4 pdu proteins have homology to the carboxysome shell protein CsoslA (Shively et al. 1998, Bobik et al. 1999). However, the functions of carboxysomes and thepdu organelles, at present, appear to be unrelated. Carboxysomes are involved in autotrophic CO2 fixation whereas the pdu organelles apparently mediate the conversion of PD to propionyl-CoA. The fact that two distinct metabolic process have been found to occur within polyhedral organelles raises two interesting questions: how many different metabolic processes occur within proteinbound organelles, and how widely distributed are such organelles in nature. Recent electron microscopy and bioinformatic studies indicate that four different metabolic processes occur within protein-bound organelles and that such structures are relatively widespread in nature. Carboxysomes are found in a number of photo- and chemoautotrophs (Shively et al. 1998, Cannon et al. 2001, Cannon et al. 2003). Electron microscopy studies have shown that Salmonella, Klebsiella, and Citrobacter form polyhedral organelles during growth on PD (Shively et al. 1998, Bobik et al. 1999,