ABSTRACT
Many protein-coding genes in bacteria are clustered together in operons which serve as transcriptional units that are co-ordinately regulated. One of the most studied of these is the lac operon in E. coli. The mechanism of induction is that the background level of β-galactosidase converts some lactose to allolactose which then acts as an inducer and turns on transcription of the lac operon. In the absence of lactose, the lac repressor of course ensures that the lac operon remains inactive. The lac operon is a good example of negative control of gene expression in that bound repressor prevents transcription of the structural genes. The lacZYA Ribonucleic acid transcript is very unstable and so degrades quickly such that further synthesis of the ß galactosidase, permease and transacetylase ceases. Positive control of gene expression is when the regulatory protein binds to Deoxyribonucleic acid and increases the rate of transcription.
