title: Mechanism of transcriptional activation by Pseudomonas aeruginosa ExsA.
abstract: ExsA is a transcriptional activator of the Pseudomonas aeruginosa type III secretion system (T3SS). The T3SS consists of >40 genes organized within 10 transcriptional units, each of which is controlled by the transcriptional activator ExsA. ExsA-dependent promoters contain two adjacent ExsA binding sites that when occupied protect the -30 to -70 region from DNase I cleavage. The promoters also possess regions bearing strong resemblance to the consensus -10 and -35 regions of sigma(70)-dependent promoters. The spacing distance between the putative -10 and -35 regions of ExsA-dependent promoters, however, is increased by 4 to 5 bp compared to that in typical sigma(70)-dependent promoters. In the present study, we demonstrate that ExsA-dependent transcriptional activation requires a 21- or 22-bp spacer length between the -10 and -35 regions. Despite the atypical spacing in this region, in vitro transcription assays using sigma(70)-saturated RNA polymerase holoenzyme (RNAP-sigma(70)) confirm that ExsA-dependent promoters are indeed sigma(70) dependent. Potassium permanganate footprinting experiments indicate that ExsA facilitates an early step in transcriptional initiation. Although RNAP-sigma(70) binds to the promoters with low affinity in the absence of ExsA, the activator stimulates transcription by enhancing recruitment of RNAP-sigma(70) to the P(exsC) and P(exsD) promoters. Abortive initiation assays confirm that ExsA enhances the equilibrium binding constant for RNAP while having only a modest effect on the isomerization rate constant.
authors.affiliation: Department of Microbiology, University of Iowa, Iowa City, Iowa 52242-1101, USA.
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Vakulskas
authors.forename: Keith M
authors.initials: KM
authors.lastname: Brady
authors.forename: Timothy L
authors.initials: TL
authors.lastname: Yahr
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D012321:DNA-Directed RNA Polymerases / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial / Q000502:physiology*; D011550:Pseudomonas aeruginosa / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation / Q000502:physiology*
chemical_list: D001426:Bacterial Proteins; C095093:ExsA protein, Pseudomonas aeruginosa; D011993:Recombinant Fusion Proteins; D015534:Trans-Activators; D012321:DNA-Directed RNA Polymerases