Organization of tcp, acf, and toxT genes within a ToxT-dependent operon.
The toxin coregulated pilus (TCP) is required for Vibrio cholerae to colonize the human intestine. The expression of the pilin gene, tcpA, is dependent upon ToxR and upon ToxT. The toxT gene was recently mapped within the TCP biogenesis gene cluster and shown to be capable of activating a tcpA::TnphoA fusion when cloned in Escherichia coli. In this study, we determined that ToxR/ToxT activation occurs at the level of tcpA transcription. ToxT expressed in E. coli could activate a tcp operon fusion, while ToxR, ToxR with ToxS, or a ToxR-PhoA fusion failed to activate the tcp operon fusion and we could not demonstrate binding of a ToxR extract to the tcpA promoter region in DNA mobility-shift assays. The start site for the regulated promoter was shown by primer extension to lie 75 bp upstream of the first codon of tcpA. An 800-base tcpA message was identified, by Northern analysis, that correlates by size to the distance between the transcriptional start and a hairpin-loop sequence between tcpA and tcpB. The more-sensitive assay of RNase protection analysis demonstrated that a regulated transcript probably extends through the rest of the downstream tcp genes, including toxT and the adjacent accessory colonization factor (acf) genes. An in-frame tcpA deletion, but not a polar tcpA::TnphoA fusion, could be complemented for pilus surface expression by providing tcpA in trans. This evidence suggests that the tcp genes, including toxT, are organized in an operon directly activated by ToxT in a ToxR-dependent manner. Most of the toxT expression under induced conditions requires transcription of the tcpA promoter. Further investigation of how tcp::TnphoA insertions that are polar on toxT expression retain regulation showed that a low basal level of toxT expression is present in toxR and tcp::TnphoA strains. Overall, these observations support the ToxR/ToxT cascade of regulation for tcp. Once induced, toxT expression becomes autoregulatory via the tcp promoter, linking tcp expression to that of additional colonization factors, exotoxin production, and genes of unknown function in cholera pathogenesis.
Brown
R C
RC
University of Tennessee, Health Science Center, Department of Microbiology and Immunology, Memphis 38163, USA.
Taylor
R K
RK
D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000235:genetics* / Q000502:physiology; D001426:Bacterial Proteins / Q000235:genetics* / Q000502:physiology; D001483:Base Sequence; D016384:Consensus Sequence; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology; D036781:Fimbriae Proteins; D010861:Fimbriae, Bacterial / Q000502:physiology*; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D008565:Membrane Proteins / Q000235:genetics* / Q000502:physiology; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes; D009876:Operon; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000096:biosynthesis; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis; D012260:Ribonucleases / Q000378:metabolism; D012689:Sequence Homology, Nucleic Acid; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology; D014158:Transcription, Genetic; D014734:Vibrio cholerae / Q000235:genetics* / Q000472:pathogenicity; D014774:Virulence
C093353:AcfA protein, Vibrio cholerae; C088551:AcfB protein, Vibrio cholerae; D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D008565:Membrane Proteins; D009097:Multienzyme Complexes; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; C085807:TcpA protein, Vibrio cholerae; D014157:Transcription Factors; C088948:acfD protein, Vibrio cholerae; C499324:toxR protein, Vibrio cholerae; C066719:toxR protein, bacteria; C066828:ToxS protein, Vibrio cholerae; D036781:Fimbriae Proteins; C075626:tcpN protein, Vibrio cholerae; D012260:Ribonucleases; C092637:Tcpj protein, Vibrio cholerae