One of the important means by which bacterial cells initiate or shut down transcription is the use of the deterrent protein associated with the control sequence, which is adjacent or overlapping with the promoter. The combination of inhibiting protein and manipulative gene inhibits transcription initiation. Taking the LAC manipulator as an example, the LACI gene, which was separated from the LAC manipulator, was encoded to deter protein. A control sequence that binds to a lactose-inhibited protein that overlaps with the promoter and is transcribed in front of about 20 bases.
Not all manipulators and promoter sites are the same relative positions, but the end result is the same, and the combination of the deterrent protein and the control sequence prevents RNA polymerase from contacting the promoter or interfering with transcription initiation. The LAC deterrent is a poly-body composed of 4 identical subunits, its two-level structure contains two alpha spirals, and two spirals are connected by several amino acids, which form a fixed angle to each other, which is known as the helix-rotation-helix sequence and is the characteristic of the DNA binding protein, It can match the main cell of some protein DNA and contact with DNA control sequence specifically. The lactose manipulation sequence is symmetric, in other words, the partial sequence is reverse-duplicated.
The four polymer structures that deter proteins identify half of the binding sites in each half, so it increases the specificity and affinity of the manipulators. Lactose-intolerant proteins are also associated with lactose, the latter being a derivative of lactase. The combination of the lactose and the deterrent protein changes the conformation of the protein, causing it to no longer bind to the manipulation of the child sites.
As a result, when lactose deficiency is inhibited, protein inactivation, manipulation of expression, and cells produce the enzymes needed for lactose metabolism. In the natural state, the lactose fermentation of Escherichia coli is the process of inducing expression, that is, only in the presence of lactose. In laboratory conditions, lactose is not a convenient inducer, because it begins to metabolize once the expression is induced, and all of them are bound to weaken its lure effect. However, its properties as inducer and substrate are separate, with the former dependent on the identification of the deterrent protein, which is identified and initiated by the galactose enzyme. Therefore, lactose analogues can be designed as inducer because they can be combined with a deterrent protein, but cannot be degraded by galactose.
IPTG, a synthetic analogue, is the placebo inducer. Conversely, some compounds are the substrate of a galactose, not its inducer, so they cannot be identified by a protein that is inhibited. X-gal is a well known non-inducible substrate, which is a color-colored substrate that is hydrolyzed by a galactose and produces blue. Therefore, in order to obtain the blue clone, the induction agent IPTG must be added in the medium.