to propionyl-CoA, thus protecting the cytoplasmic contents. This model is supported by the identification of the diol dehydratase and propionaldehyde dehydrogenase enzymes among the organelle proteins. In addition to these two enzymes, both components of the putative diol dehydratase reactivating factor and an adenosyltransferase were identified; however, no reductase was identified. A scenario where B12 use is limited by the lack of reduced Ado-B 12 precursors is therefore a viable possibility for the regulation mechanism of diol dehydratase activity, and consequently, aldehyde production.

      In the model, the first two steps of the pathway, the conversion of PD to

propionaldehyde and its subsequent oxidation to propionyl-CoA, are proposed to occur within the polyhedral organelle while reduction to propanol and further conversion of propionyl-CoA to propionic acid are thought to occur outside of the organelle. Alternatively, the enzymes required for these latter reactions could have been missed due to low quantities; however, this seems unlikely since it seems reasonable that the pathway enzymes would be present in roughly equal amounts. A third possibility is that these proteins are associated with the outside of the organelle and are lost during purification due to either interruption of ionic bonds or hydrophobic interactions by use of sodium chloride or detergent. Further study will be needed to determine the location of these enzymes. If one assumes that the placement of the enzymes is correct, the PduQ protein may serve to convert aldehyde which has leaked out of the polyhedral organelles into the less toxic metabolite, propanol, which is then subsequently excreted from the cell.

                              Future Experimentation

      The development of a purification procedure for the polyhedral organelles opens the door for many future experiments. Biochemical studies of organelle function and mechanisms are now possible. Specifically, the comparison of activities of enzymes