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Haemophilus ducreyi causes chancroid, a genital ulcer disease that facilitates the transmission of human immunodeficiency virus type 1. In humans,H. ducreyi is surrounded by phagocytes and must adapt to a hostile environment to survive. To sense and respond to environmental cues, bacteria frequently use two-component signal transduction (2CST) systems. The only obvious 2CST system inH. ducreyi is CpxRA; CpxR is a response regulator, and CpxA is a sensor kinase. Previous studies by Hansen and coworkers showed that CpxR directly represses the expression ofdsrA , thelspB -lspA2 operon, and theflp operon, which are required for virulence in humans. They further showed that CpxA functions predominantly as a phosphatasein vitro to maintain the expression of virulence determinants. Since acpxA mutant is avirulent while acpxR mutant is fully virulent in humans, CpxA also likely functions predominantly as a phosphatasein vivo . To better understand the role ofH. ducreyi CpxRA in controlling virulence determinants, here we defined genes potentially regulated by CpxRA by using RNA-Seq. Activation of CpxR by deletion ofcpxA repressed nearly 70% of its targets, including seven established virulence determinants. Inactivation of CpxR by deletion ofcpxR differentially regulated few genes and increased the expression of one virulence determinant. We identified a CpxR binding motif that was enriched in downregulated but not upregulated targets. These data reinforce the hypothesis that CpxA phosphatase activity plays a critical role in controllingH. ducreyi virulencein vivo . Characterization of the downregulated genes may offer new insights into pathogenesis.

Activation of CpxRA in Haemophilus ducreyi Primarily Inhibits the Expression of Its Targets, Including Major Virulence Determinants