REGULATION OF THE NDH GENE OF ESCHERICHIA-COLI BY INTEGRATION HOST FACTOR AND A NOVEL REGULATOR, ARR

The ndh gene of Escherichia coil encodes the non-proton-translocating NADH dehydrogenase II. Expression of the ndh gene is subject to a comp lex network of regulatory controls at the transcriptional level. Under anaerobic conditions ndh is repressed by the regulator of fumarate an d nitrate reduction (FNR). However, in the absence of FNR, ndh express ion is activated by the amino acid response regulator (Arr) during ana erobic growth in rich medium. Expression of the ndh gene varies during the growth cycle in response to the intracellular concentration of th e heat-stable DNA-binding protein, Fis. In this work two additional he at-stable proteins, integration host factor (IHF) and the histonelike protein HU were found to interact with the ndh promoter. IHF was shown to bind at three sites centred at +26, -17 and -58 in the ndh promote r (K-d = 10(-8) M), to prevent open-complex formation and to repress n dh transcription in vitro. Studies with an ndh-lacZ fusion confirmed t hat IHF represses ndh expression in vivo. Two putative binding sites f or Arr, which overlap the two FNR boxes in the ndh promoter, were iden tified. Studies with the FNR-activated and amino-acid-inducible aspara ginase II gene (ansB) showed that IHF and a component of the Arr-conta ining fraction (but not HU) interact with the corresponding ansB promo ter.