Abstract
H10407 is a strain of enterotoxigenic
Escherichia coli
(ETEC) that utilizes CFA/I pili to adhere to surfaces of the small intestine, where it elaborates toxins that cause profuse watery diarrhea in humans. Expression of the CFA/I pilus is positively regulated at the level of transcription by CfaD, a member of the AraC/XylS family. DNase I footprinting revealed that the activator has two binding sites upstream of the pilus promoter
cfaAp
. One site extends from positions −23 to −56, and the other extends from positions −73 to −103 (numbering relative to the transcription start site of
cfaAp
). Additional CfaD binding sites were predicted within the genome of H10407 by computational analysis. Two of these sites lie upstream of a previously uncharacterized gene,
cexE
. In vitro DNase I footprinting confirmed that both sites are genuine binding sites, and
cexEp
::
lacZ
reporters demonstrated that CfaD is required for the expression of
cexE
in vivo. The amino terminus of CexE contains a secretory signal peptide that is removed during translocation across the cytoplasmic membrane through the general secretory pathway. These studies suggest that CexE may be a novel ETEC virulence factor because its expression is controlled by the virulence regulator CfaD, and its distribution is restricted to ETEC.
