Summary

Gene expression refers to whether a product is produced from a gene. Gene regulation refers to the control of gene expression, whether a gene is expressed under appropriate conditions.

Genes can be regulated at multiple levels, although this chapter focuses primarily on transcriptional regulation. Proteins called activators positively regulate gene expression by increasing the rate of transcription. Proteins called repressors negatively regulate gene expression by decreasing the rate of transcription.

Activators and repressors can work together on a single promoter to tightly control expression of a gene. The lac operon is an example of this, regulated by both an activator and a repressor. For proper gene regulation, a gene must be expressed at appropriate times and also turned off at appropriate times – it’s not just a matter of being able to turn the gene on! Constitutive mutants of the operator – O^C result in an operon that is always on, but this is not properly regulated because it cannot be turned off when there is no lactose.

One single protein can be both an activator and a repressor, depending on where it binds in the genome. An example of this is the lambda repressor, which controls the switch between lytic and lysogenic growth of bacteriophage λ.

In prokaryotes, but not eukaryotes, the action of the ribosome during translation can affect whether or not a gene is fully transcribed. The trp operon is an example of that.