Difference between revisions of "Part:BBa K566002"

Revision as of 16:32, 28 September 2011

Biphasic switch

The biphasic switch combines positive and negative regulation through a single input. It is turned ON by low lambda cI concentrations and OFF by high cI concentrations(1), behavior shown in figure 1.

Figure 1. The Biphasic Switch. Behavior of wild type (wt) Lambda’s pRM in presence of the OL region placed 3.8 kb apart for this assay. LacZ gene is placed under pRM’s control. At low concentrations of cI, lacZ expression is stimulated. At high concentrations of cI, lacZ gets repressed. Figure taken and modified from Dodd BI et al. (2001) Gene Dev 15:3013–3022.

Usage and Biology

There are two well-known sets of cI binding sites in lambda (OR and OL) spaced 2.4 kb apart and composed of three operators each (OR1:OR2:OR3; OL1:OL2:OL3) (2). cI has a higher affinity for OR1 and OR2 operators, where binding positively regulates the pRM promoter. In presence of the OL set, cI octamerise specifically binding to the two sets of operators (OR and OL) at the same time and therefore forming a DNA loop (3) as shown in figure 2. Such structure stabilizes cI's binding to OR3, allowing pRM's repression at cI high concentrations.

Figure 2. (a) EM picture of a circular relaxed plasmid with a large DNA loop formed by cI. Two tetramers on each DNA circle interact to form an octamer and thereby loop the DNA into a structure resembling a figure of eight. (b)Close up cartoon representing cI octamers occupying the OL1, OL2, OR1, and OR2 operators. OR3 is to be occupied as [cI] increases. The lower portion shows the sequence of PRM and OR3, the latter between the -10 and -35 where cI cooperative binding inhibits transcription. Figures taken and modified from: Révet B et al. (1999) Curr. Biol. 9: 151–154 and Dodd IB et al.(2001) Genes Dev 15:3013–3022, respectively.

Sequence and Features