So we all know a little bit about how genes are expressed in animal cells, but what about bacteria? Despite the fact that they are unicellular organisms, they still need to control which proteins are produced, and when they are produced. Like how we discussed earlier that it would be useless if your toes starting producing pepsinogen, bacteria need to conserve energy and it would be of no use to them whatsoever to start producing enzymes to digest lactose, when there is no lactose to digest, or there’s plenty of glucose to go round everyone!
The first example of gene expression in bacteria we’ll look a is the lac operon. Now just to start us off you should know that bacteria will only begin using lactose as an energy source when there is no glucose present (kind of like how we as humans, don’t start using fat as an energy source unless there are no carbohydrates to use), and so the enzymes which digest lactose only need to be produced in certain environmental conditions, so why bother wasting energy producing proteins which aren’t needed!
There are three gene sequences which code for the enzymes which digest lactose (β-galactosidase, β-galactoside permease and thiogalactoside transacetylase, but lets not get to caught up with that, it’s not too important to know the names right now) and they are located next to each other, and are referred to as lacZ, lacY, and lacA. When these genes are transcribed by mRNA, a single mRNA strand is produced, and this strand is referred to as polycistronic, as it codes for more than one protein. When lactose is not present, the lac repressor protein binds to the operator region (the start point for mRNA transcription) and this prevents RNA polymerase from binding to the DNA, and so prevents the genes being transcribed, and therefore the proteins being produced. However when lactose is present, lactose binds to the repressor protein, changing it’s conformation in such a way the does not allow it to bind to the operator region, and so the genes are transcribed and the proteins are produced!
The lac operon is said to be an inducible operon, as gene transcription is induced by the presence of lactose.
The trp operon works in a very similar way, except it is referred to as a repressible operon, because gene expression is repressed by the presence of high levels of the amino acid tryptophan. It works under the same principles as negative feedback in eukaryotes. “What, there’s loads of that stuff been produced!? Better stop for a bit until we get rid of some of it!”.
When low levels of tryptophan are present, the conformation of the repressor protein does not allow it to bind to the operator region, therefore the genes which code for tryptophan are transcribed as RNA polymerase can bind to the DNA. When high levels of tryptophan are present, the organism not longer needs to produce it, and two molecules of tryptophan bind to the repressor protein, and change it’s conformation in such a way which means that it can now bind to the DNA and prevent transcription.
Smart little things bacteria, eh?