CHAPTER 5
                                  CONCLUSIONS

      The discovery of homologues to carboxysome proteins in the eut and pdu operons and the subsequent observation of polyhedral organelles in cells grown under conditions which induce these operons, sparked interest in determining the role of these organelles in B312-dependent metabolism. Prior to the work conducted in this dissertation little was known about the polyhedral organelles except that they were produced in cells grown on PD. This dissertation focused on elucidating the structure and function of the polyhedral organelles involved in the AdoB12-dependent degradation of PD.

        Conditions Required for the Formation of the Polyhedral Organelles

      The first part of this study (Chapter 2) investigated the conditions required and

identified some of the genes involved in the formation of the polyhedral organelles in £. enterica. Previous studies on the regulation of thepdu operon demonstrated that thepdu genes are induced aerobically during growth on poor carbon sources but are maximally expressed during anaerobic growth on a poor carbon source in the presence of PD (Bobik et al. 1992, Chen et al. 1995). Studies described here show that polyhedral organelles are formed under similar growth conditions.

      To examine the role of polyhedral organelles in the degradation of PD, a time course of organelle formation was performed. Using electron microscopy, polyhedral organelles were observed during growth of S enterica on PD and were found to appear just prior to entrance into the log phase of growth. The number of polyhedral organelles appeared highest during mid-log phase, and even though the numbers dwindled in the late