The Ames test is used to determine if an external agent (such as a chemical) can act as a mutagen. In this test, several different his- strains of Salmonella typhimurium are used. Each strain contains a different class of mutation in the his gene. Each mutant is exposed to the chemical to determine if the chemical can revert the mutation back to wild type (his+), and at what frequency.

(a) How can such prototrophic revertants be identified?

(b) In this experiment, three different strains of Salmonella were used to determine the mutagenic properties of three different chemicals. The following table lists the type of mutation in each his mutant and the number of revertants obtained with the addition of each of the three chemicals. Are any of the chemicals putative mutagens? If so, what type of mutations do they cause? Justify your answer.

Strain Type of his-mutation Water Chemical1 Chemical2 Chemical3
Strain1 T to G base substitution 2 2000 3 5
Strain 2 G to A base substitution 1 1 4 1700
Strain 3 Single base deletion 3 2 4 3