GENE REGULATION: LAC OPERON

Introduction

Prokaryotic as well as eukaryotic organisms possess different mechanisms to control the regulation of their genes. Cells need to be efficient and avoid wasting energy in the production of unnecessary proteins. Most of these mechanims take place at the transcriptional level. Gene regulation can be negative or positive. In negative regulation, a repressor molecule binds to the operator of an operon and terminates transcription. In positive regulation, an activator interacts with the RNA polymerase in the promoter region to initiate transcription.

The lactose operon is an example of negative regulation. When Escherichia coli cells are growing in a medium containing lactose as the only carbon source, some of the lactose is converted to allolactose. Allolactose acts as an inducer and turns on the lac operon. In the presence of glucose and the absence of lactose, allolactose is not produced and the lac operon is turned off. Since glucose is a readily available energy source, cells won't use lactose until they consume all the glucose. When both glucose and lactose are present in the medium, the operon is also off. In this case, the operon is regulated by catabolite repression.

In order to study the expression of the different genes in the lac operon, cells are mutagenized. They can be exposed to UV light or to chemical mutagens. Mutants can be separated in groups depending on where the mutations are located. Mutations can occur within the same gene coding for a specific protein or not within a gene coding for a specific protein but within another region in the DNA sequence. The cis-trans test which is also called a complementation test can be used to detect if a gene is acting in cis or in trans. When a gene acts in cis to a mutation, it affects only the expression of genes in the same DNA molecule where the mutation occurs. When a gene acts in trans to a mutation, it affects the expression of genes on other DNA molecules not containing the mutation.

Some mutants of the lac operon are called constitutive mutants. In these mutants, the structural lac genes are transcribed even when lactose is not present in the medium. Two constitutive mutants of the lac operon are lacI and lacOc. Another type of mutant is the superrepressor mutant lacIs.


To find more information about the lactose operon, visit the links page included in this web site. Also, a page with the references used to make this web site is also included.

Return to Home