The Lactose Operon

Draw a graph to illustrate usage of glucose and lactose in E. coli.

Okay, you probably know the drill by now. I'm not going to draw anything, because I'm lazy, and besides there are plenty of better graphs on Google Images. Just google "growth of E coli glucose lactose" and you will find several that show what I'm about to describe.

The growth of E. coli in glucose and lactose occurs in two stages, and thus is known as "diauxic growth." Firstly, E. coli consumes glucose, as that is its preferred source of energy. Once the glucose has been consumed, E. coli begins to transcribe the genes needed for metabolism of lactose. While this is occurring, growth temporarily stops or slows down considerably. Once the genes have been transcribed, E. coli begins consuming lactose, and its growth starts again.

List the enzymes of the lactose operon and their functions.

The lactose operon produces two main enzymes: beta-galactosidase and lactose permease. Lactose permease transports lactose into the cell, whereas beta-galactosidase hydrolyses lactose into glucose and galactose. Beta-galactosidase also has a side pathway in which lactose is converted to allolactose. This is the inducer for negative regulation, as we shall see in a bit. The lactose operon also has a section coding for transacetylase, the function of which is still unclear.

Describe the lactose operon and its negative regulation.

The lactose operon consists of six regions: IPOZYA. The I part codes for the repressor protein. P and O are promoter and operator, respectively- see my previous post for definitions of these terms. Z codes for beta-galactosidase, Y codes for lactose permease and A codes for transacetylase.

The negative regulation of the lactose operon involves the repressor coded by the I region of the operon. (Remember, negative regulation always involves a repressor.) This time, instead of a co-repressor helping the repressor bind to the DNA, there is an inducer that prevents the repressor from binding to the DNA. In this case, the inducer is allolactose. When allolactose binds to the repressor, the repressor does not bind to the DNA, allowing transcription to take place. (Glucose also needs to be either not present or present only in low levels, but I'll cover that in my next post when I talk about positive regulation.)

I'm going to talk a little bit more about the repressor protein itself because it's interesting enough to have slides devoted to it. The lac repressor is a tetramer- that is, it's made up of four polypeptide chains bound together via mainly noncovalent bonds. It has two identical binding sites that can bind onto the DNA. The lac operator actually has three binding sites: O1 (the main site), which is between P and Z, O2, which is after Z, and O3, which is before P. The repressor binds to O1 and either O2 or O3. A loop is formed, which contains the sites that are usually recognised by RNA polymerase. Hence, binding of the repressor to form a DNA loop prevents binding by RNA polymerase and therefore also prevents transcription.

Another interesting point to make about negative regulation of the lactose operon is the "chicken and egg" problem. Allolactose is required to remove the repressor, allowing transcription of the mRNA for beta-galactosidase and lactose permease. However, beta-galactosidase is required for the production of allolactose in the first place! How do cells get around this? Well, in my previous post I mentioned that prokaryotic genes generally cannot be turned off, but rather have their activity reduced to a low basal level. Hence there is always a little beta-galactosidase around to convert lactose to allolactose.

Explain the terms inducer, inducible, on-off regulation and diauxic growth.

An inducer, as I've mentioned before, is a protein that binds to a repressor in order to prevent the repressor from binding to DNA, thus allowing transcription of the gene. An inducible protein is a protein produced in this manner.

On-off regulation is the switching on or off of genes due to the presence or absence of repressors or activators.

Diauxic growth is growth in two stages, such as the growth of E. coli in the presence of glucose and lactose.