The central message of Fig. 19.1 is that the regulation of gene expression occurs through a hierarchy of regulatory mechanisms acting at different levels (and usually at multiple levels) from DNA to protein. Gene regulation in prokaryotes is simpler than gene regulation in eukaryotes since DNA is not packaged into nucleosomes, mRNA is not processed, and transcription and translation are not separated by a nuclear envelope. In prokaryotes, expression of a protein-coding gene entails transcription of the gene into messenger RNA and translation of the messenger RNA into protein. Each of these levels of gene expression is subject to regulation.

Because gene regulation in prokaryotes is simpler than gene regulation in eukaryotes, prokaryotes have served as model organisms for our understanding of how genes are turned on and off. In this section, we consider in more detail how gene expression is regulated at the level of transcription in bacteria and in viruses that infect bacteria. We focus on two well-studied systems: (1) the regulation of genes in the intestinal bacterium Escherichia coli that allows proteins needed to utilize the sugar lactose to be produced only when lactose is present in the environment and only when it is the best nutrient available, and (2) the regulation of genes in a virus that infects E. coli that controls whether the virus integrates its DNA into the bacterial host or lyses (breaks open) the cell. In both cases, specific genes are turned on and off in response to environmental conditions.