Cooperativity in long-range gene regulation by the λ CI repressor (original) (raw)

  1. Ian B. Dodd1,3,
  2. Keith E. Shearwin1,
  3. Alison J. Perkins1,
  4. Tom Burr2,
  5. Ann Hochschild2, and
  6. J. Barry Egan1
  7. 1Discipline of Biochemistry, School of Molecular and Biomedical Science, University of Adelaide, South Australia 5005, Australia;2Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, Massachusetts 02115, USA

Abstract

Effective repression of cI transcription from _P_RM by the bacteriophage λ CI repressor requires binding sites (_O_L) located 2.4 kb from the promoter. A CI tetramer bound to O_L1.O_L2 interacts with a tetramer bound near _P_RM (O_R1.O_R2), looping the intervening DNA. We previously proposed that in this CI octamer:DNA complex, the distant _O_L3 operator and the weak _O_R3 operator overlapping _P_RM are juxtaposed so that a CI dimer at _O_L3 can cooperate with a CI dimer binding to _O_R3. Here we show that _O_L3 is necessary for effective repression of _P_RM and that the repressor at _O_L3 appears to interact specifically with the repressor at _O_R3. The _O_L3-CI-_O_R3 interaction involves the same CI interface used for short-range dimer-dimer interactions and does not occur without the other four operators. The long-range interactions were incorporated into a physicochemical model, allowing estimation of the long-range interaction energies and showing the lysogenic state to be ideally poised for CI negative autoregulation. The results establish the λ system as a powerful tool for examining long-range gene regulatory interactions in vivo.

Footnotes