The H-NS protein represses transcription of the eltAB operon, which encodes heat-labile enterotoxin in enterotoxigenic Escherichia coli, by binding to regions downstream of the promoter.

Heat-labile enterotoxin, a major virulence determinant of enterotoxigenic Escherichia coli, is encoded by the eltAB operon. To elucidate the molecular mechanism by which the heat-stable nucleoid-structural (H-NS) protein controls transcription of eltAB, the authors constructed an eltAB-lacZ transcriptional fusion and performed beta-galactosidase analysis. The results showed that H-NS protein exerts fivefold repression on transcription from the eltAB promoter at 37 degrees C and 10-fold repression at 22 degrees C. Two silencer regions that were required for H-NS-mediated repression of eltAB expression were identified, both of which were located downstream of the start site of transcription. One silencer was located between +31 and +110, the other between +460 and +556, relative to the start site of transcription, and they worked cooperatively in repression. DNA sequences containing the silencers were predicted to be curved by in silico analysis and bound H-NS protein directly in vitro. Repression of eltAB transcription by H-NS was independent of promoter strength, and the presence of H-NS protein did not affect promoter opening in vitro, indicating that repression was achieved by inhibiting promoter clearance or blocking transcription elongation, probably via DNA looping between the two silencers.