Abstract
ExsA activates type III secretion system (T3SS) gene expression in
Pseudomonas aeruginosa
and is a member of the AraC family of transcriptional regulators. AraC proteins contain two helix-turn-helix (HTH) DNA binding motifs. One helix from each HTH motif inserts into the major groove of the DNA to make base-specific contacts with the promoter region. The amino acids that comprise the HTH motifs of ExsA are nearly identical to those in LcrF/VirF, the activators of T3SS gene expression in the pathogenic yersiniae. In this study, we tested the hypothesis that ExsA/LcrF/VirF recognize a common nucleotide sequence. We report that
Yersinia pestis
LcrF binds to and activates transcription of ExsA-dependent promoters in
P. aeruginosa
and that plasmid-expressed ExsA complements a
Y. pestis lcrF
mutant for T3SS gene expression. Mutations that disrupt the ExsA consensus binding sites in both
P. aeruginosa
and
Y. pestis
T3SS promoters prevent activation by ExsA and LcrF. Our combined data demonstrate that ExsA and LcrF recognize a common nucleotide sequence. Nevertheless, the DNA binding properties of ExsA and LcrF are distinct. Whereas two ExsA monomers are sequentially recruited to the promoter region, LcrF binds to promoter DNA as a preformed dimer and has a higher capacity to bend DNA. An LcrF mutant defective for dimerization bound promoter DNA with properties similar to ExsA. Finally, we demonstrate that the activators of T3SS gene expression from
Photorhabdus luminescens
,
Aeromonas hydrophila
, and
Vibrio parahaemolyticus
are also sensitive to mutations that disrupt the ExsA consensus binding site.
