Characterization of two operons that encode components of fructose-specific enzyme II of the sugar:phosphotransferase system of Streptococcus mutans

Abstract

Three genes, designated as fruC, fruD and fruI, were predicted to encode polypeptides homologous to fructose-specific enzyme II (II(Fru)) of the phosphoenolpyruvate-dependent sugar:phosphotransferase system, and were cloned from Streptococcus mutans, the primary etiological agent of human dental caries. The fruC and fruD genes encoded domains BC and domain A of II(Fru), respectively. The fruI gene encoded IICBA(Fru). Northern hybridization and slot blot analysis showed that expression of fruI was inducible by sucrose and fructose, while fruCD were expressed constitutively and at much lower levels. Inactivation of either fruI or fruCD alone, or of both fruCD and fruI, had no major impact on growth on fructose at a concentration of 0.5% (w/v). However, when the strains were grown with 0.2% fructose as the sole carbohydrate source, a significant decrease in the growth rate was seen with the fruCD/fruI double mutants. Assays of sugar:phosphotransferase activity showed that the fruCD/fruI double mutants had roughly 30% of the capacity of the wild-type strain to transport fructose via the phosphoenolpyruvate-dependent sugar:phosphotransferase system. Xylitol toxicity assays indicated that the inducible fructose permease was responsible for xylitol transport.

Global Transcriptional Analysis of Streptococcus mutans Sugar Transporters Using Microarrays

“…In fact, previous studies indicated that fructose was transported into the cell via an inducible PTS and a constitutive PTS (11,40). Wen and colleagues (40) have characterized transcription of both transporters.…”

“…Recently published data (42) concerning fructose metabolism confirmed our results. The previously reported fructose inducible operon (SMU.113 to -116) (40) showed very low transcrip-…”

“…RT-PCR data also revealed increased transcription of SMU.872 (encoding EII Fru ) and decreased transcription of SMU.2047 (encoding EII Mal/Glu ) in the presence of sucrose. EII Fru is a part of the PTS that has been characterized as a constitutive fructose transporter (11,40), so it was not surprising to find further increase of transcription of the EII Fru gene in sucrose, since the fructose moiety of this disaccharide can be released extracellularly. Alternatively, the possibility exists that EII Fru might transport sucrose.…”

“…Comparison of this sequence to the genome database showed that one of the five highly expressed EIIs is EII Suc . In addition to the inducible fructose PTS, S. mutans also possesses a constitutive fructose PTS (11,40). Comparison of its sequence to the genome database revealed that EII Fru was also one of the five highly transcribed transporters.…”

Responses of Cariogenic Streptococci to Environmental Stresses

“…The mechanism by which EIIAB Man regulates these virulence genes remains unknown, but it is becoming evident that the PTS plays a central role in gene regulation in oral streptococci (Abranches et al, 2003). Other PTS enzymes of S. mutans, including a fructose PTS enzyme, have been shown to regulate expression of catabolism of polysaccharides (Wen et al, 2001). Of note, inactivation of the inducible fructose-PTS of S. gordonii resulted in a biofilm-defective phenotype (Mitrakul et al, 2003), again suggesting a global regulatory function for the PTS.…”