EXPRESSION OF THE GENES-CODING FOR THE ESCHERICHIA-COLI INTEGRATION HOST FACTOR ARE CONTROLLED BY GROWTH-PHASE, RPOS, PPGPP AND BY AUTOREGULATION

Transcriptional control of the himA and the himD/hip genes coding for the two subunits of the integration host factor (IHF) was investigated , The promoters for the two genes were identified by the use of primer extension and S1 analysis. Expression from both promoters was found t o increase as the cells enter stationary phase, Mutation in rpoS, know n to be induced upon entry to stationary phase, dramatically reduced t he growth-phase response of the himA P4 promoter but had only a small effect on the induction of the himD/hip promoter, The increased activi ty of both promoters required the presence of the relA and spoT genes, suggesting that ppGpp plays a major role in the response to stationar y phase. An artificial increase in ppGpp in exponentially growing cell s induced a rapid increase in himA P4 and himD/hip mRNA levels. Experi ments with a mutant defective in rpoS showed that the response of the himA P4 promoter to high ppGpp levels was greatly reduced while that o f himD/hip was only slightly affected. Therefore, it seems that differ ent mechanisms involving RpoS and ppGpp regulate the growth-phase resp onse of the two promoters, We propose that the effect of ppGpp on himA P4 is mediated via RpoS whereas the himD/hip promoter is affected by ppGpp independently of RpoS. Expression of the himD/hip and himA genes was found to be subject to negative autoregulation. IHF-binding sites , implicated in auroregulation, were found to overlap both the himD/hi p and himA P4 promoters. An additional IHF-binding site was found upst ream of the himD/hip promoter, All three sites show low binding affini ty to IHF suggesting that autoregulation can take place only after suf ficiently high levels of IHF accumulate in the cell.