Analyses of the DNA-binding and transcriptional activation properties of ExsA, the transcriptional activator of the Pseudomonas aeruginosa exoenzyme S regulon.

ExsA has been implicated as a central regulator of exoenzyme S production by Pseudomonas aeruginosa. In this study, the DNA-binding and transcriptional activation properties of ExsA were investigated. ExsA was produced and purified as a fusion protein, MALA3A2, which was shown to bind specifically to promoter regions that regulated transcription of the exoenzyme S trans-regulatory locus (pC) and a locus located directly downstream of exsA (pD). Previously, MALA3A2 was shown to bind the exoS 5' PstI-NsiI region, which contained two independent but coordinately regulated (ExsA-mediated) promoters, pS' (now termed pORF1) and pS. DNase I footprint analysis of the promoter regions bound by ExsA revealed a common protected consensus sequence of TXAAAAXA. The consensus sequence was located -51 to -52 bp upstream of the transcriptional start sites for pD, pS, and pORF1. Promoter fusion, DNA-binding, and mutagenesis analysis indicated that the consensus sequence was important for transcriptional activation. Each ExsA-controlled promoter region contained at least two consensus sites in close proximity, similar to the arrangement of half-sites seen in AraC-controlled (Escherichia coli) or VirF-controlled (Yersinia enterocolitica) promoters. However, the results of this study suggested that only one consensus site was required in the exoenzyme S (pS) or ORF1 promoter (pORF1) to initiate transcription. These data suggest that members of the exoenzyme S regulon can be defined as possessing an ExsA consensus element which maps at bp -51 or -52 relative to the transcriptional start site.