The biosynthetic pathway for histidine, which has several parallels with the shikimic acid pathway (Figure 21.13 and Figure 21.14) for aromatic amino acids, is shown in Figure 21.17. Ten individual reactions are involved. In enteric bacteria, all structural genes for the enzymes responsible for histidine synthesis are linked on the histidine operon in the same order as the reactions of the pathway.

The Ames test, which is used to search for mutagens (substances causing mutations) in the environment, is based on the histidine operon. In the test, bacteria with a mutation that disables the operon from making functional histidine pathway enzymes (auxotrophic bacteria) are treated with a suspected mutagen. The mutation rate can be easily assayed by counting the number of bacteria that mutate to being able to make histidine (prototrophs). This is done quite simply by plating the treated bacteria onto a medium lacking histidine. Only bacteria that have mutated and regained the ability to synthesize histidine will grow on the medium without histidine.

There is a very high correlation between the compounds known to be carcinogenic in animals and those found to be mutagens in the Ames test. Thus, the Ames test provides a quick and inexpensive way to search for suspectted carcinogens in the environment.