Transcriptional control in prokaryotes
In prokaryotes, control of transcription is by far the most important level of gene expression regulation. There is very little control in the subsequent stages of protein synthesis.
Control of transcription is simpler in prokaryotes than in eukaryotes and for that reason is much better understood.
In prokaryotes, groups of genes with similar functions are expressed together in an operon.
The expression of genes in an operon is controlled by a single promoter sequence.
The lac-operon is a classic model of gene expression in prokaryotes.
Expression is initiated at the promoter sequence (where RNA polymerase binds). The operator region controls whether RNA polymerase can bind to the promoter. Activator and repressor proteins (types of transcriptional factors) are able to bind to it.