AUTHOR=Liu Aaron C. , Thomas Nikhil A. 

TITLE=Transcriptional profiling of Vibrio parahaemolyticus exsA reveals a complex activation network for type III secretion

JOURNAL=Frontiers in Microbiology

VOLUME=6

YEAR=2015

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2015.01089

DOI=10.3389/fmicb.2015.01089

ISSN=1664-302X

ABSTRACT=<p><italic>Vibrio parahaemolyticus</italic> (Vp) is a marine halophilic bacterium that is commonly associated with oysters and shrimp. Human consumption of contaminated shellfish can result in Vp mediated gastroenteritis and severe diarrheal disease. Vp encodes two type 3 secretion systems (T3SS-1 and T3SS2) that have been functionally implicated in cytotoxicity and enterotoxicity respectively. In this study, we profiled protein secretion and temporal promoter activities associated with <italic>exsA</italic> and <italic>exsB</italic> gene expression. <italic>exsA</italic> is an AraC-like transcriptional activator that is critical for activating multiple operons that encode T3SS-1 genes, whereas <italic>exsB</italic> is thought to encode an outer membrane pilotin component for T3SS-1. The <italic>exsBA</italic> genetic locus has two predicted promoter elements. The predicted <italic>exsB</italic> and <italic>exsA</italic> promoters were individually cloned upstream of <italic>luxCDABE</italic> genes in reporter plasmid constructs allowing for <italic>in situ</italic>, real-time quantitative light emission measurements under many growth conditions. Low calcium growth conditions supported maximal <italic>exsB</italic> and <italic>exsA</italic> promoter activation. <italic>exsB</italic> promoter activity exhibited high basal activity and resulted in an <italic>exsBA</italic> co-transcript. Furthermore, a separate proximal <italic>exsA</italic> promoter showed initial low basal activity yet eventually exceeded that of <italic>exsB</italic> and reached maximal levels after 2.5 h corresponding to an entry into early log phase. <italic>exsA</italic> promoter activity was significantly higher at 30°C than 37°C, which also coincided with increased secretion levels of specific T3SS-1 effector proteins. Lastly, bioinformatic analyses identified a putative expanded ExsA binding motif for multiple transcriptional operons. These findings suggest a two wave model of Vp T3SS-I induction that integrates two distinct promoter elements and environmental signals into a complex ExsA activation framework.</p>