Regulation of expression of the fructan hydrolase gene of Streptococcus mutans GS-5 by induction and carbon catabolite repression

The polymers of fructose, levan and inulin, as well as sucrose and raffinose, are substrates for the product of the fruA gene of Streptococcus mutans GS-5. The purpose of this study was to characterize the DNA immediately flanking fruA, to explore the regulation of expression of fruA by the carbohydrate source, and to begin to elucidate the molecular basis for differential expression of the gene. Located 3' to fruA was an open reading frame (ORF) with similarity to β-fructosidases which was cotranscribed with fruA. A transcriptional initiation site, located an appropriate distance from an extended -10-like promoter, was mapped at 165 bp 5' to the fruA structural gene. By the use of computer algorithms, two overlapping, stable stem-loop sequences with the potential to function as rho-independent terminators were found in the 5' untranslated region. Catabolite response elements (CREs), which have been shown to govern carbon catabolite repression (CCR) by functioning as negative cis elements in gram-positive bacteria, were located close to the promoter. The levels of production of fruA mRNA and FruA were elevated in cells growing on levan, inulin, or sucrose as the sole carbohydrate source, and repression was observed when cells were grown on readily metabolizable hexoses. Deletion derivatives containing fusions of fruA promoter regions, lacking sequences 5' or 3' to the promoter, and a promoterless chloramphenicol acetyltransferase gene were used (i) to demonstrate the functionality of the promoter mapped by primer extension, (ii) to demonstrate that CCR of the fru operon requires the CRE that is located 3' to the promoter region, and (iii) to provide preliminary evidence that supports the involvement of an antitermination mechanism in fruA induction.