Molecular analysis of the promoter region of the Clostridium difficile toxin B gene that is functional in Escherichia coli

Binary toxin locus analysis in Clostridium difficile

The objective of this study was to compare full binary toxin loci (CDTloc) sequences from a collection of Clostridium difficile isolates in an effort to further understand the regulation of the binary toxin (CdtAB) and its putative regulator (CdtR). Sequences from different ribotypes and toxinotypes were analysed phylogenetically and for polymorphisms, non-sense mutations, promoter features and signal sequences. Expression of cdtA, which was also representative of cdtB expression, was measured by quantitative PCR (qPCR). Several consensus promoter features and various polymorphisms were identified including a non-sense mutation identified in a ribotype 078 cdtR gene that is predicted to result in a severely truncated protein. Despite this mutation, cdtA expression was still detected by qPCR. Dendrograms based on total sequences indicated that isolates belonging to the same ribotype shared the greatest similarity within the binary toxin locus. Although cdtR is thought to be involved in regulation of cdtA expression, a cdtR non-sense mutation did not inhibit expression of cdtA, suggesting that either the truncated protein is functional or another regulator of the binary toxin exists.

Sequence analysis of a new open reading frame located in the pathogenicity locus of Clostridium difficile strain 8864

Strain 8864 is a natural isolate of Clostridium difficile that is toxin B-positive and toxin A-negative. Recent work showed that there is a genetic rearrangement occurring at the pathogenicity locus (PaLoc) of the bacteria. Our investigation in the PaLoc region revealed an open reading frame (tcdF) of 543 bp DNA not reported before. This tcdF could encode a putative polypeptide of 22 kDa. Although no peptide homology was found with other known proteins, we postulate that it could be a novel protein because not only highly consensus ribosome-binding and promoter sequences were found upstream of tcdF, transcript was also identified at the region occupied by tcdF.

The enzymatic domain of Clostridium difficile toxin A is located within its N-terminal region

Clostridium difficile, an anaerobic pathogen encountered in human enteric disease, produces two major virulence factors, toxins A and B, which are members of a clostridial family of large cytotoxins. These are glucosyltransferases, which use a UDP-sugar as co-substrate to glucosylate and inactivate small GTPases of the Rho or Ras families, culminating in cytotoxicity. Clinically, toxin A is perhaps the most important family member, because it causes major tissue damage in the course of disease, leading to a potentially lethal, pseudomembranous colitis. The location of the enzymatic domain of toxin A and mechanistic details of its action are not yet known, so we wished to localize this domain using gene deletion constructions from the full-length gene and by monitoring glucosylation activity of encoded protein products. Toxin A deletions were obtained by successively truncating the C-terminal coding region. These were transformed into E. coli, cell lysates were prepared and they were assayed for their ability to glucosylate Rho A protein, using an in vitro enzymatic assay. We report that the UDP-glucose binding site, the catalytic site for glucose transfer and the Rho A interaction site occur within the first 659 N-terminal amino acids of toxin A, i.e., less than 25% of the length of holotoxin A. Localization of the enzymatic domain of toxin A to these 659 N-terminal amino acids should greatly simplify studies on mechanistic details of this clinically important toxin.