significantly better than that observed in the insertion mutant suggesting that abnormal downstream expression was the culprit in the complementation studies conducted in the pduA::TPOP insertion mutant. The occurrence of some aberrant polyhedral organelles was later found to be due to the inadvertent deletion of the start codon for the pduB gene in this strain (see below).

      The pduA deletion provided a nonpolar mutant to study the phenotype of a

polyhedral organelle minus strain. Growth studies indicated that an extended period of "interrupted" growth occurred in the mutant and not in the wild type. Physiological studies conducted on the pduA mutant indicated that this lag was related to the amount of PD included in the growth medium suggesting that buildup of a toxic intermediate may be the cause of the "interrupted" growth. Additional physiological studies demonstrated that the pduA mutant grew at faster initial rates than the wild type when supplied with low amounts of B12 suggesting that the polyhedral organelles may act as B12 barriers. This finding suggests that the polyhedral organelles may play a role in limiting aldehyde toxicity by moderating the rate of aldehyde production via control of B12 accessibility.

                      Purification of the Polyhedral Organelles Like the carboxysome, study of the polyhedral organelles of S. enterica would be greatly facilitated by a purification procedure that provides homogenous, stable organelles. The third part (Chapter 4) of this study dealt with the development of such a purification scheme. As a starting point, the procedure developed by Cannon et al. for the purification of carboxysomes from Halothiobacillus neapolitanus was used (Cannon and Shively 1983). Like this procedure, sonication proved to be the best method for breaking cells for polyhedral organelle purification. Subsequent centrifugation however resulted in pelleting of the polyhedral organelles with the cell debris and unbroken cells