Coat protein synthesis during sporulation of Bacillus subtilis: immunological detection of soluble precursors to the 12,200-dalton spore coat protein.

Antibody specific to the 12,200-dalton spore coat protein of Bacillus subtilis was used to detect the synthesis of cross-reacting material during sporulation. Cross-reacting protein was first detected by immunoprecipitation after 4 h of development and represented at least 1 to 2% of the total soluble protein synthesis at 5.5 h. A polypeptide of 21,000 daltons was detected in immunoprecipitates by gel electrophoresis. This polypeptide did not accumulate in sporulating cells and was rapidly turned over at the time of coat deposition. In contrast, a 32,000-dalton polypeptide reacted with antibody when unlabeled cell protein was denatured with sodium dodecyl sulfate, separated by gel electrophoresis, and transferred to nitrocellulose paper. This polypeptide was not detected during cell growth or the first 3.5 h of development but was found to accumulate in sporulating cells at 5.5 h. The lack of detection of this polypeptide by immunoprecipitation of undenatured protein indicates that the antigenic sites which cross-reacted with antibody to the 12,200-dalton protein sequence were not exposed unless the molecular conformation was altered. The 32,000-dalton protein may be a primary translation product which is proteolytically processed into mature spore coat protein via a 21,000-dalton intermediate.