Describe the concept of ‘operons’ in gene expression.

The term “operon” was coined by Jacob and Monod, who characterized the first defined classical operon, the lac operon, in Escherichia coli. Operons can be termed as, a cluster of coordinately regulated genes.

It includes structural genes (generally encoding enzymes), regulatory genes (encoding, e.g. activators or repressors) and regulatory sites (such as promoters and operators). The type of control is defined by the response of the operon when no regulatory protein is present. In the case of negative control, the genes in the operon are expressed unless they are switched off by a repressor protein. Thus the operon will be turned on constitutively (the genes will be expressed) when the repressor in inactivated. In the case of positive control, the genes are expressed only when an active regulator protein, e.g. an activator, is present. Thus the operon will be turned off when the positive regulatory protein is absent or inactivated, Inducible operons are turned on in response to a metabolite (a small molecule undergoing metabolism) that regulates the operon. E.g. the lac operon is induced in the presence of lactose through the action of a metabolic by-product. allolactose). Repressible operons are switched off in response to a small regulatory molecule. E.g., the trp operon is repressed in the presence of tryptophan. Although lac is an inducible operon, we will see conditions under which it is repressed or induced (via derepressing)

 Regulatory region contains: 

Promoter: binds RNA polymerase holoenzyme to initiate transcription.

Operator: A DNA sequence that can bind a repressor Repressor: protein that prevent RNA polymerase from initiating transcription Transcription is inhibited when repressors bind to their operators Regulatory gene: genes coding for an activator or repressor protein Promoter of the regulatory gene.

 Coding region: 

Often contains multiple genes for several different proteins. 

These genes are called structural genes 

These genes are either all turned on or all turned off.