Bacterial cells regulate gene expression in response to environmental changes through transcriptional control mechanisms. The lac and arg operons are two examples that are regulated in Escherichia coli. The lac operon encodes enzymes for lactose metabolism and is induced in the presence of lactose. The arg operon encodes enzymes for arginine biosynthesis and is repressed in the presence of arginine. Operons allow functionally related genes to be coordinately regulated at the level of transcription through repressors and inducers binding to operator sites on the DNA.

1. Microbial Genetics The Operon: Regulation of bacterial gene expression

4. Regulation of gene expression Constitutive Enzymes – No regulation Enzyme Induced Enzyme Repressed

14. Arrangement of the lac operon