Homodimerization and Binding of Specific Domains to the Target DNA Are Essential Requirements for HlyU To Regulate Expression of the Virulence Gene rtxA1 , Encoding the Repeat-in-Toxin Protein in the Human Pathogen Vibrio vulnificus

The virulence genertxA1 , encoding the repeat-in-toxin protein, plays an essential role in the pathogenesis ofVibrio vulnificus infections. Expression of this gene is controlled by the HlyU regulator by direct contact of the DNA upstream of thertxA1 toxin operon acting as a derepressor of the H-NS protein. The crystal structure suggests that HlyU forms a homodimerin vitro . However, knowledge of the biological implications of these findingsin vivo is limited. In this work, we endeavored to dissect, using genetic and biochemical approaches, the domains of this protein that are essential for homodimer formation and the interaction of HlyU with the target DNA. We identified that residues L18, N22, R25, S54, Q55, L57, W59, R61, K70, and Y77 are essential for the HlyU protein binding to the DNA and that amino acids L17 and L91 are important for HlyU dimerization. We also determined that HlyU homodimer formation is an essential requirement for binding to the upstream region of thertxA1 operon and is the key feature in relieving the H-NS repression ofrtxA1 transcription.