|Abstract:||Bacteroides thetaiotaomicron, a gram-negative obligate anaerobe, appears to utilize starch by first binding the polymer to its surface, then translocating it into the periplasmic space. Several genes have been identified that encode enzymes or outer membrane proteins involved in starch utilization. These have been called sus genes, for starch utilization system. Previous studies have shown that sus structural genes are regulated at the transcriptional level, and their expression is induced by maltose. We report here the identification and characterization of a gene, susR, which appears to be responsible for maltose regulation of the sus structural genes. The deduced amino acid sequence of SusR protein had a helix-turn-helix motif at its carboxyterminal end and this region had highest sequence similarity to the corresponding regions of known transcriptional activators. A disruption in susR eliminated expression of all known 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 binding of SusR to maltose might be the step that activates SusR. Three sus structural genes, susA, susB and susC, are located immediately upstream of susR. These genes are organized into two transcriptional units, one containing susA and another containing susB and susC. susA was expressed at a lower level than susBC, and susA expression was more sensitive to the gene dosage of susR than the susBC operon. An unexpected finding was that increasing the number of copies of susR in B. thetaiotaomicron increased the rate of growth on starch. Thus, the level of SusR in the cell appears to be a limiting factor for growth on starch.