The Structural Genes

The structural genes direct synthesis of cellular proteins through messenger RNA (mRNA) and determine the sequence of amino acids in the proteins synthesized. There as many structural genes in an operon as there are proteins or polypeptides under common control. Each structural gene may be controlled independently and transcribe a separate mRNA molecule, or all the structural genes of an operon may form one long polycistronic or polygenic mRNA molecule. Polygenic mRNAs are common in bacteria and bacteriophages. All know eukaryotes, however, contain only a single functional site for initiating protein synthesis.

The lactose (lac) operon of the bacterium Escherichia coli has been particularly well studied. It is a small segment of a circular DNA molecule which constitutes most of the genetic material of the bacterium. It has three structural genes, z, y and a, which transcribe one long polycistronic mRNA molecule. This controls the synthesis of three different proteins (enzymes), β-galactosidase, galactoside permease and galactoside transacetylase. These consist of 4, 1 and 2 units, respectively. β-galactosidase is the enzyme that splits lactose into glucose and galactose. Galactoside permease facilities the entry of lactose into the cell. Galactoside acetuylase appears in small quantities upon lactose induction.



The initiation codon of structural gene z is TAC (corresponding to AUG or mRNA). It is located 10 base pairs away from the end of the operator gene.
In the bacterium Salmonella typhimurium the histidine operon (histidon) has 10 structural genes.