THE TRANSCRIPTIONAL ACTIVATOR HLYU OF VIBRIO-CHOLERAE - NUCLEOTIDE-SEQUENCE AND ROLE IN VIRULENCE GENE-EXPRESSION

HlyU upregulates expression of the haemolysin, HlyA, of Vibrio cholera e. DNA sequence analysis indicates that HlyU is an 11.9 kDa protein co ntaining a putative helix-turn-helix motif and belonging to a family o f small regulatory proteins, including NolR (Rhizobium meliloti), SmtB (Synechococcus PCC 7942) and ArsR (plasmids R773, Escherichia coli; p l258, Staphylococcus aureus; and pSX267, Staphylococcus xylosus). An h lyU mutant was constructed by insertional inactivation, and found to b e deficient in the production of both the haemolysin and a 28 kDa secr eted protein. The mutant was assessed for virulence in the infant mous e cholera model, revealing a 100-fold increase in the LD50. This sugge sts that HlyU promotes expression of virulence determinant(s) in vivo.