A BACTEROIDES-THETAIOTAOMICRON OUTER-MEMBRANE PROTEIN THAT IS ESSENTIAL FOR UTILIZATION OF MALTOOLIGOSACCHARIDES AND STARCH

Previous studies suggested that the first step in utilization of starc h by Bacteroides thetaiotaomicron was was binding of the polysaccharid e to the cell surface, followed by translocation of the polysaccharide across the outer membrane into the periplasm. In this study, we repor t the molecular characterization of a gene that encodes an outer membr ane protein that is essential for utilization of both maltooligosaccha rides and starch. The gene, susC, encoded a protein of 115.3 kDa. Anti bodies were raised against SusC, and the outer membrane location of Su sC could be confirmed by Western blot (immunoblot) analysis. SusC had a possible signal sequence of between 20 and 39 amino acids, depending on which N-terminal methionine initiates the start of the protein. It also had some features typical of well-characterized outer membrane p roteins from members of the family Enterobacteriaceae, such as a termi nal phenylalanine residue and a region in the amino portion of the pro tein thought to be involved in stabilizing the protein in the outer me mbrane. The amino acid sequence, together with results of gene disrupt ion experiments, suggested that SusC was not an amylolytic enzyme. Tra nscriptional fusion experiments, using beta-glucuronidase as a reporte r group, showed that expression of susC was maltose regulated at the t ranscriptional level. This is the first molecular characterization of a B. thetaiotaomicron outer membrane protein involved in maltooligosac charide and starch utilization.