Animation 16.1: The lac Operon

INTRODUCTION

The bacterium E. coli has an efficient mechanism for metabolizing lactose. Three proteins that are important in lactose metabolism are all encoded in a single expressible unit of DNA, called the lac operon. The bacterium does not waste energy expressing these proteins if lactose is not present in the growth medium. It only makes these proteins when lactose is available to be metabolized.

In this tutorial, we examine how the presence of lactose turns on the expression of these lactose-metabolizing genes.

CONCLUSION

The bacterium E. coli can grow in cultures supplemented with a variety of energy sources, including the sugar lactose. However, to use lactose, E. coli must first alter its own metabolism. The bacterium must turn on several structural genes found in the lac operon that are required for lactose metabolism.

When lactose is not present, the structural genes of the lac operon are not expressed. A repressor, which is always present in the cell, binds to the lac operon and prevents transcription by blocking the passage of RNA polymerase. However, when lactose is present in the environment, it enters the cell and some of it is converted to a similar molecule called allolactose. Allolactose is the inducer, and when it binds to the repressor it causes the receptor to change shape, such that the repressor can no longer bind to the lac operator. In this case, RNA polymerase can proceed unimpeded through the operon and transcribe the genes needed for lactose metabolism.

The lac operon is an inducible system, meaning that the system is turned off until an inducer arrives on the scene. Other operons, such as the trp operon, work in the opposite way: this system expresses genes in the operon until a repressor becomes activated and turns the expression off.

Textbook Reference: Key Concept 16.1 Prokaryotic Gene Expression Is Regulated in Operons