An operon consists of an operator, promoter, regulator, and structural genes. The regulator does not have to be adjacent to other genes in the operon. If the repressor protein is removed, transcription may occur. Structure of a typical operon. Image from W.H. Freeman and Sinauer Associates, used by permission.
Operons are either inducible or repressible according to the control mechanism.
Seventy-five different operons controlling 250 structural genes have been identified for E. coli . Bacteria have flagella with a different microtubule structure than the flagella of eukaryotes. Cell walls of bacteria contain peptidoglycan instead of the cellulose found in cell walls of plants and some algae. Ribosomes are the structures in cells where proteins are assembled. Bacterial ribosomes have different sized ribosomal subunits than do eukaryotes.