Plasmid replication initiator protein TrfA represses the host type III secretion system in <i>Pseudomonas aeruginosa</i>

<i>Pseudomonas aeruginosa</i> is an environmental bacterium and opportunistic human pathogen capable of causing a wide range of infections in humans. The type III secretion system (T3SS) plays a key role in the pathogenesis of acute <i>P. aeruginosa</i> infections. Expression of the T3SS is controlled by a master activator ExsA, whose transcription is driven by its operon promoter P<i>_exsC</i> as well as its own promoter P<i>_exsA</i>. Here, we found that plasmid replication initiation protein TrfA acts as a repressor for the T3SS gene expression in <i>P. aeruginosa</i>. Production of the TrfA reduced the expression of the T3SS genes and the bacterial pathogenicity in a mouse acute pneumonia model. Further analysis revealed that the expression of <i>trfA</i> resulted in upregulation of the novel gene PA5530, which mediates reduced cAMP production and decreased the expression of T3SS in PAK. In addition, we demonstrate that TrfA regulates the T3SS by directly binding to and modulating the P<i>_exsC</i> promoter. Of note, the clinical plasmid harboring the <i>trfA</i> gene also leads to downregulation of T3SS in <i>P. aeruginosa</i>. Overall, the results from this study reveal the effect of the plasmid replication initiation protein on the expression of its host chromosomal genes.IMPORTANCEAs extrachromosomal elements, plasmids are well known for their role in conferring advantageous attributes to hosts, including antibiotic resistance, heavy metal resistance, the ability to degrade xenobiotics, and osmotolerance. Although several host chromosomally encoded proteins are required for the replication and maintenance of plasmids in their hosts, few proteins from plasmids have been reported to affect the host chromosomal gene expression. In this study, we identified TrfA, a plasmid replication initiation protein, as a novel repressor of the T3SS in <i>P. aeruginosa</i>. We also elucidated the regulatory mechanism of T3SS repression mediated by the TrfA. The significance of this work is in the identification of the replication initiation protein of a naturally occurring plasmid from <i>P. aeruginosa</i> functioning as a regulator of the expression of chromosomal genes in its host <i>P. aeruginosa</i>.