Positive Regulation of the Lactose Operon

I covered negative regulation of the lactose operon in my previous post... now it's time to talk about positive regulation!

List the components important in positive regulation of the Lac operon.

The main components involved in positive regulation are the Lac promoter, the activator and cAMP (cyclic AMP). The promoter on the Lac operon is a weak promoter, so under normal conditions, RNA polymerase will not recognise it well. The activator, CAP, binds to cyclic AMP to form a CAP-cAMP dimer. This dimer binds to a region of DNA near the promoter, allowing RNA polymerase to bind to the DNA more readily. This, in turn, allows for transcription of the genes.

Describe the positive regulation of the Lac operon.

The main thing to understand here is that cAMP levels are inversely proportional to glucose levels. As glucose levels rise, cAMP levels drop, and when glucose levels drop, cAMP levels rise. As the function of the Lac operon is to allow the cell to metabolise lactose in the absence of glucose, you might expect that low glucose levels, and therefore high cAMP levels, will allow for transcription of the gene. This is exactly what happens: as I mentioned above, cAMP can bind to the activator protein CAP, forming a CAP-cAMP dimer that can bind near the promoter, inducing the transcription of the genes.

List 3 regulatory DNA-binding proteins in bacteria.

Easy: I've already mentioned three in this post and in my previous posts about gene regulation. TrpR is a repressor that binds to the tryptophan operon, LacI is a repressor that binds to the lactose operon and CAP is an activator that binds to the lactose operon.

Understand how proteins can bind to specific regions of DNA, and the roles of some conformational changes caused by this binding.

Proteins can bind to the outside of DNA, particularly in the major groove of the double helix. Amino acids on the protein can form hydrogen bonds with the sides of bases in the double helix. (Proteins cannot actually insert themselves into the double helix.) A common DNA-binding motif is the helix-turn-helix motif. One of the helices is called the recognition helix as it helps the protein to recognise the major groove of the DNA. Generally these proteins exist as dimers (proteins with two subunits), with the two recognition helices separated by exactly one turn of the DNA helix.

Binding of proteins to the DNA can cause conformational changes to the DNA, which in turn can assist or prevent the binding of RNA polymerase. For example, the CAP-cAMP dimer bends the DNA by 90 degrees, facilitating the binding of RNA polymerase to the DNA. Also, as mentioned in my previous post, other proteins can create loops that prevent the binding of RNA polymerase to the DNA.