Regulation of expression of the region 3 promoter of the Escherichia coli K5 capsule gene cluster involves H-NS, SlyA, and a large 5' untranslated region.

Escherichia coli group 2 capsule gene clusters are temperature regulated, being expressed at 37 degrees C but not at 20 degrees C. Expression is regulated at the level of transcription by two convergent promoters, PR1 and PR3. In this paper, we show that regulation of transcription from PR3 involves a number of novel features including H-NS, SlyA, and a large 741-bp 5' untranslated region (UTR). H-NS represses transcription from PR3 at 20 degrees C and binds both 5' and 3' of the transcription start site. The 3' downstream regulatory element (DRE) was essential for temperature-dependent H-NS repression. At 37 degrees C, SlyA activates transcription independent of H-NS but maximal transcription requires H-NS. The UTR is present between the transcription start site and the first gene in the operon, kpsM. We demonstrate that the UTR, as well as containing the H-NS DRE, functions to moderate the extent of transcription that reaches kpsM and allows the binding of antitermination factor RfaH.