was then used to generate polyclonal antiserum. Immunolabeling with this antiserum demonstrated that the PduA protein localized to the organelle periphery, suggesting it was a component of the shell. Additional studies demonstrated that S. entericapduA null mutants did not make polyhedral organelles, and when grown on propanediol minimal medium they exhibited a period of arrested growth. Subsequent physiological tests suggested that the organelles might function as a B12 barrier, and the arrested growth observed in organelle mutants may result from increased production of a toxic intermediate.

      In order to determine the constituent proteins of the polyhedral organelles, a

purification scheme was developed to obtain stable, homogenous preparations. After purification, preparations were separated using one and two-dimensional gel electrophoresis. The major proteins of the organelle were identified using peptide mass fingerprinting, N-terminal sequencing, and immunoblotting. A total of 14 proteins was identified, including four enzymes: coenzyme B12-dependent-diol dehydratase, CoAdependent propionaldehyde dehydrogenase, adenosyltransferase, and a putative twocomponent diol dehydratase-reactivating factor. Based on these findings, a model is proposed wherein the polyhedral organelles serve to prevent cellular toxicity by channeling and sequestering propionaldehyde as well as by moderating the rate of its production.