L. Swint-Kruse, K.S. Matthews, in Encyclopedia of biological Chemistry (Second Edition), 2013

Multiple Sites/Multiple Targets – DNA Looping

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In enhancement to the primary operator, LacO, displayed in Figure 1, two ‘pseudo-operator’ sequences are existing within the lac operon sequence and contribute to repression. The DNA sequences of the pseudo-operators are very similar, yet not identical, to LacO and also are bound by LacI an ext weakly. The visibility of 2 DNA-binding web page in LacI protein tetramer argued a mechanism whereby pseudo-operators could enhance repression – one LacI tetramer can bind two different operators and generate a looped DNA structure. Experimental proof for DNA looping has been acquired from a selection of laboratories. Recent evidence indicates the the angle between the two dimers have to open for loop formation to occur, as shown in Figure 4. This looped structures are highly stabilized, accountancy for the significant repression that lacZYA expression it was observed in bacterial cells. Indeed, DNA containing many operator sequences and with the supercoiling density characteristic that E. Coli exhibits a half-life for the facility that exceeds 2 days. However, also these looped complexes respond swiftly (in less than 30 s) to the visibility of inducer sugars, allowing quick adaptation come an external lactose source that might be transient.


Figure 4. Looped DNA structure. The teal blue bent line depicts the lac operon DNA (with shading to show nearness come observer), which consists of three feasible LacI-binding sites (two the which, O1 and also O2, are presented bound to LacI). The pseudo-operator sequence O2 is located within the lacZ gene, and the primary operator succession O1 overlaps the promoter sequence for the lacZYA metabolic gene (Figure 1). Tetrameric LacI is shown at the bottom the the figure as simultaneously communicating with O1 and O2. This framework loops the DNA and also generates a complicated with an extremely high stability.

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Keep in mind that the dimers within a LacI tetramer different and adopt a bigger angle in between them as soon as looping in between O1 and O2 than in the absence of looping (i.e., the structure shown in Figure 3(a)). The need for flexibility in between the dimers because that looping to occur is supported by speculative evidence.