et al. 1998, Shively et al. 1998, Kaplan and Reinhold 1999, Badger and Price 2003), and the carboxysomes of Halothiobacillus neapolitanus (which are the best studied) consist of a protein shell composed of at least six different proteins which encases most of the cell's ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) (Cannon and Shively 1983, Holthuijzen et al. 1986, English et al. 1994, Baker et al. 1999, Baker et al. 2000). In contrast, the organelles of S. enterica do not contain RuBisCO, but instead consist of Ado-B12-dependent diol dehydratase and a protein shell composed in part of the PduA protein and other unidentified proteins (Bobik et al. 1999, Havemann et al. 2002). It has been proposed that the S. enterica organelles function to minimize aldehyde toxicity by moderating propionaldehyde production through control of Ado-B12 availability. However, this function has not been established. Moreover, the mechanism of the S. enterica organelles has not been investigated and a great deal remains to be learned about their structure.

     Here we report the purification and structural characterization of the unusual

organelles involved in PD degradation by S. enterica. The analyses performed included one and two-dimensional electrophoresis, immunoblotting, N-terminal sequencing and protein mass fingerprinting via Matrix Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS). By these methods, 15 proteins of the organelles were identified. These included Ado-B12-dependent diol dehydratase (PduCDE), CoA-dependent propionaldehyde dehydrogenase (PduP), adenosyltransferase (PduO), the large (PduG) and small (PduH) subunits of the putative diol dehydratasereactivating factor, the PduA shell protein and six additional, probable structural proteins (pduBB'JKTU) as well as one unidentified protein. These findings are consistent with a