A significant difference between bacterial and eukaryotic gene control lies in the organization of functionally related genes. As we saw in the introduction to this chapter, many bacterial genes that have related functions are clustered together and are under the control of a single promoter. These genes are often transcribed together into a single mRNA molecule. A group of bacterial structural genes that are transcribed together, along with their promoter and additional sequences that control their transcription, is called an operon. The operon regulates the expression of the structural genes by controlling transcription, which, in bacteria, is usually the most important level of gene regulation.