The essentials
Gene expression can be regulated at several levels in eukaryotes: transcription (formation of the primary transcript), processing (splicing), transport of mRNA from nucleus to cytoplasm, and translation. In prokaryotes, control of the rate of transcription initiation is the main site. A regulatory gene produces a repressor or activator that affects whether RNA polymerase can transcribe the structural genes.
The lac operon in E. coli was described by François Jacob and Jacques Monod. An operon is a set of genes transcribed together and controlled by a common promoter and operator. The lac operon has a regulatory gene i (producing the repressor) and three structural genes:
- z codes for β-galactosidase, which breaks lactose into glucose and galactose.
- y codes for permease, which increases permeability of the cell to lactose.
- a codes for transacetylase.
When lactose is absent, the repressor (made constitutively by the i gene) binds the operator and prevents RNA polymerase from transcribing the operon. When lactose (or allolactose) is present, it acts as an inducer: it binds the repressor and inactivates it, so the repressor cannot bind the operator, and transcription proceeds. This is negative regulation. A little lactose enters the cell initially through a low level of permease present.
Key points
- Inducer for the lac operon: lactose (allolactose).
- The operon is called inducible because a substrate switches it on.
- Glucose or galactose cannot act as inducers of the lac operon.
Common mistakes
- The repressor gene i is not part of the structural genes; "i" stands for inhibitor.