EFFECT OF REGULATORY PROTEIN-LEVELS ON UTILIZATION OF STARCH BY BACTEROIDES-THETAIOTAOMICRON

Bacteroides thetaiotaomicron, a gram-negative obligate anaerobe, appea rs to utilize starch by first binding the polymer to its surface and t hen translocating it into the periplasmic space. Several genes that en code enzymes or outer membrane proteins involved in starch utilization have been identified. These have been called sus genes, for starch ut ilization system. Previous studies have shown that sus structural gene s are regulated at the transcriptional level and their expression is i nduced by maltose. We report here the identification and characterizat ion of a gene, susR, which appears to be responsible for maltose-depen dent regulation of the sus structural genes. The deduced amino acid se quence similarity to the corresponding regions of known transcriptiona l activators. A disruption in susR eliminated the expression of all kn own sus structural genes, as expected if susR encoded an activator of sus gene expression. The expression of susR itself was not affected by the growth substrate and was not autoregulated, suggesting that bindi ng of SusR to maltose might be the step that activates SusR. Three sus R-controlled structural genes, susA, susB, and susC, are located immed iately upstream of susR. These genes are organized into two transcript ional units, one containing susA and another containing susB and susC. susA was expressed at a lower level than susBC, adn susA expression w as more sensitive to the gene dosage of susR than was that of the susB C operon. An expected finding was that increasing the number of copies of susR in the cell appears to be a limiting factor for growth on sta rch.