Ra. Burne et al., Regulation of expression of the fructan hydrolase gene of Streptococcus mutans GS-5 by induction and carbon catabolite repression, J BACT, 181(9), 1999, pp. 2863-2871
The polymers of fructose, levan and inulin, as well as sucrose and raffinos
e, 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 fla
nking fruA, to explore the regulation of expression of fruA by the carbohyd
rate 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 beta-fructosidases which was cotranscribed with fruA. A
transcriptional initiation site, located an appropriate distance from an e
xtended -10-like promoter, was mapped at 165 bp 5' to the fruA structural g
ene. By the use of computer algorithms, two overlapping, stable stem-loop s
equences 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 funct
ioning as negative cis elements in gram-positive bacteria, were located clo
se to the promoter. The levels of production of fruA mRNA and FruA were ele
vated in cells growing on levan, inulin, or sucrose as the sole carbohydrat
e source, and repression was observed when cells were grown on readily meta
bolizable hexoses. Deletion derivatives containing fusions of fruA promoter
regions, lacking sequences 5' or 3' to the promoter, and a promoterless ch
loramphenicol acetyltransferase gene were used (i) to demonstrate the funct
ionality of the promoter mapped by primer extension, (ii) to demonstrate th
at CCR of the fru operon requires the CRE that is located 3' to the promote
r region, and (iii) to provide preliminary evidence that supports the invol
vement of an antitermination mechanism in fruA induction.