Groups of genes coding for related proteins are arranged in units known as operons. 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. Images 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 . Both repression and induction are examples of negative control since the repressor proteins turn off transcription. Images from W.H. Freeman and Sinauer Associates, used by permission. Plasmids carry between 2 and 30 genes. Some seem to have the ability to move in and out of the bacterial chromosome.