The lac Operon - an inducible system
The trp Operon - a repressible system
Introduction
Control of gene expression is at the level of transcription. If a gene is not transcribed then the gene product and ultimately the phenotype will not be expressed. We are now going to consider two systems of control of gene expression in the E. coli cell. Both of these systems are concerned with the production of enzymes involved in cell metabolism, but each exhibits a different type of control.
Induction - the production of a specific enzyme (or set of enzymes) in response to the presence of a substrate
Repression - the cessation of production of a specific enzyme (or set of enzymes) in response to an increased level of a substrate
All of the genes which encode the enzymes necessary for the pathway are found next to each other on the E. coli chromosome. One key feature of both systems to be discussed is that a single mRNA is transcribed with multiple translation stop codons. The proteins that can be translated from the mRNA are the enzymes required for a specific pathway. This type of mRNA is called a polycistronic mRNA and is totally unique to prokaryotes.
Copyright © 1997. Phillip McClean