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Pathogenesis of enterotoxigenic Escherichia coli (ETEC) and Shigella flexneri relies predominantly on members of the AraC/XylS family of transcriptional regulators, Rns (or its homolog, CfaD) and MxiE, respectively. Rns/CfaD regulate the expression of pili, which allow the bacteria to attach to the intestinal epithelium. Better understanding of the role Rns plays in virulence was attained by expanding our knowledge of the Rns regulon, revealing that it functions as an activator of cexE, a previously uncharacterized gene. By in vitro DNase I footprinting two Rns-binding sites were identified upstream of cexEp, both of which are required for full activation of cexE. The amino terminus of CexE also contains a secretory signal peptide that is removed during translocation to the periplasm. Though the function of CexE remains unknown, these studies suggest that CexE is a novel ETEC virulence factor since it is regulated by Rns/CfaD. In Shigella flexneri, the expression of a subset of virulence genes (including, ipaH9.8 and ospE2) is dependent upon the activator MxiE and a cytoplasmic chaperone IpgC. To define the molecular mechanism of transcriptional activation by this chaperone-activator pair, an in vitro pull down assay was performed revealing that MxiE specifically interacts with IpgC in a complex. Additionally, IpgC recognizes three polypeptide regions in MxiE: within MxiE(1-46), MxiE(46-110) and MxiE(196-216). Furthermore, it seems that MxiE and IpgC regulate transcription of ipaH9.8 and ospE2 promoters differently. In the bacterium, the formation of the MxiE-IpgC complex is initially prevented because IpgC is sequestered in individual complexes with effector proteins, IpaB and IpaC. Upon contact with an eukaryotic host cell the effector proteins are secreted, thereby freeing IpgC to form a complex with MxiE and activate the expression of virulence genes. This new characterization of the role of Rns and MxiE in virulence gene regulation in ETEC and S. flexneri, respectively will give new insights into the pathogenesis of the regulators.