Mapping of a gene that regulates hemolysin production in Vibrio cholerae

The transcriptional activator HlyU of Vibrio cholerae: nucleotide sequence and role in virulence gene expression

HlyU upregulates expression of the haemolysin, HlyA, of Vibrio cholerae. DNA sequence analysis indicates that HlyU is an 11.9 kDa protein containing a putative helix-turn-helix motif and belonging to a family of small regulatory proteins, including NoIR (Rhizobium meliloti), SmtB (Synechococcus PCC 7942) and ArsR (plasmids R773, Escherichia coli; pI258, Staphylococcus aureus; and pSX267, Staphylococcus xylosus). An hlyU mutant was constructed by insertional inactivation, and found to be deficient in the production of both the haemolysin and a 28 kDa secreted protein. The mutant was assessed for virulence in the infant mouse cholera model, revealing a 100-fold increase in the LD50. This suggests that HlyU promotes expression of virulence determinant(s) in vivo.

Transcription of the Vibrio cholerae haemolysin gene, hlyA, and cloning of a positive regulatory locus, hlyU

Transcription of the Vibrio cholerae hlyA gene, which encodes a cytotoxic haemolysin, has been investigated. The hlyA transcript initiates 430 nucleotides (nt) upstream of the translational start site. hlyA-cat transcriptional fusion constructs were active in V. cholerae but not in Escherichia coli. An hlyA-cat fusion was used to select, from a V. cholerae O17 plasmid library, a clone that could activate the hlyA promoter in E. coli. This regulatory locus has been designated hlyU. hlyU appears to be distinct from the previously described hlyR locus.

Genetics of Vibrio cholerae and its bacteriophages

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Hemolysin production and cloning of two hemolysin determinants from classical Vibrio cholerae

The hemolytic activity of 20 classical and 3 El Tor strains of V. cholerae O1 was examined phenotypically and genetically. The El Tor strains lysed bovine, chicken, human, rabbit, and sheep erythrocytes (RBCs), while the classical strains lysed only chicken and rabbit RBCs. The assay was done with RBCs in Tris-NaCl buffer, since phosphate-buffered saline was found to inhibit hemolytic activity. Hemolytic activity in culture supernatants from El Tor strains was more sensitive to heat inactivation than that in supernatants from the classical strain 395. A gene library of strain 395 was examined for hemolytic activity, and two distinct hemolytic clones were identified. One clone appeared identical to the previously cloned hemolysin structural gene from El Tor V. cholerae, while the other did not hybridize to the El Tor hemolysin probe, had a unique restriction enzyme digestion pattern, and encoded a hemolysin whose activity differed from that of the El Tor hemolysin clones. We suggest that the hemolysin specified by the determinant originally cloned from an El Tor vibrio be designated hemolysin I and the second hemolysin, cloned from the classical vibrio, be designated hemolysin II.