ABSTRACT
An overview Many protein-coding genes in bacteria are clustered together in operons, which serve as transcriptional units that are coordinately regulated. It was Jacob and Monod in 1961 who proposed the operon model for the regulation of transcription. The operon model proposes three elements:
z An operator site, which is a DNA sequence that regulates transcription of the structural genes
The lac operon One of the most studied operons is the lac operon in E. coli. This codes for key enzymes involved in lactose metabolism: galactoside permease (also known as lactose permease; it transports lactose into the cell across the cell membrane) and b-galactosidase (which hydrolyzes lactose to glucose and galactose). It also codes for a third enzyme, thiogalactoside transacetylase. Normally E. coli cells make very little of any of these three proteins but when lactose is available, it causes a large and coordinated increase in the amount of each enzyme to be synthesized. Thus each enzyme is an inducible enzyme and the process is called induction. The mechanism is that the few molecules of b-galactosidase in the cell before induction convert the lactose to allolactose, which then turns on transcription of these three genes in the lac operon. Thus allolactose is an inducer. Another inducer of the lac operon is isopropylthiogalactoside (IPTG). Unlike allolactose, this inducer is not metabolized by E. coli and so is useful for experimental studies of induction.
