Typical Features Of Operons
The genes of an operon are usually functionally related. Genes are the basic unit of biological information, and consist of specific segments of deoxyribonucleic acid (DNA). The segments of DNA that constitute a gene consist of distinctive sets of nucleotide pairs located in a discrete region of a chromosome that encodes a particular protein. Within an operon, the genes encode proteins that execute related functions. For example, the five genes of the tryptophan (trp) operon in E. coli each encode one of the enzymes necessary for the biosynthesis of the amino acid tryptophan from a metabolite called chorismate. This condition is mimicked in many bacteria.
Exceptions do occur; the genes of some operons may lack an obvious functional relationship. For example one operon in E. coli contains one gene that encodes a ribosomal protein S21 (rpsU), another that encodes DNA primase (dnaG), and one that encodes the sigma subunit of RNA polymerase (rpoD). The protein products of these genes are all involved in starting up the synthesis of macromolecules, but beyond that they have no obvious functional relationships to one another. Nonetheless, the clustering of these genes and their common regulation qualify them to be treated as elements of a single operon.
Another common feature of operons is that their genes are clustered on the bacterial chromosome. This chromosome is a large circular molecule of DNA. The genes of an operon are arranged in a consecutive and linear fashion at a specific location on the bacterial chromosome.
In the case of the lactose utilization (lac) operon of E. coli, three genes necessary for the successful utilization of the disaccharide lactose, a common sugar found in milk, are arranged in a linear fashion on the chromosome (see Figure 1). The lacZ gene, which encodes the lactose-degrading enzyme, β-galactosidase, is directly followed by the lacY gene, which encodes a membrane protein, called lactose permease, that allows the entry of lactose into the cell. The lacY gene is immediately followed by the lacA gene, which encodes the thiogalactoside acetyltransferase enzyme that detoxifies lactose-related compounds that might be toxic to the cell. The linear arrangement of these functionally related genes is a hallmark of an operon. The arrangement is significant because the proteins made from these genes will all be easily turned on in concert, so that lactose metabolism proceeds rapidly and efficiently.