0.1 Basic principles of genetics  (Page 7/19)

1) Back mutation - true wild type

2) Intragenic suppressor - compensating mutation in same gene

3) Extragenic suppressor - compensating mutation in different gene

These possibilities can be distinguished in that a revertant that arose by suppression will still carry the starting mutation (now masked by the suppressor mutation), whereas a back mutation will produce a true wild type phage. The general test is to cross the revertant to wild type and to note whether mi- recombinants are observed. A back mutation crossed to wild type will not produce any mi- progeny, whereas a revertant that results from an extragenic suppressor will produce many mi- recombinants. Intragenic suppressors will produce an intermediate result that sometimes can be difficult to distinguish from a back mutation in practice. For example, an intragenic suppressor that lies very close to the original mi- mutation may be able to produce mi- recombinants in principle, but these recombinants may be too rare to be readily observed.

Nonsense suppressor

A mutation that changes the anticodon to CUA allows the mutant tRNAser to recognize a UAG codon and insert tryptophan when a UAG codon appears in a coding sequence.

Recognition of UCG (serine codon) Recognition of UAG(stop codon) by wild type tRNAser by amber suppressor mutant tRNAser (*)

mRNA: 5’——————UCG——————3’ 5’————UAG––——————3’

The presence of an amber suppressing mutation is usually designated Su+ whereas a wild-type (nonsuppressing) strain would be designated Su-.

Example: Pam designates an amber (nonsense) mutation in the λ phage P gene, which is required for λ phage DNA replication. When λ Pam phage are grown on E. coli with an amber suppressor (Su+), the phage multiply normally; but when λ Pam phage infect a nonsuppressing host (Su–), the phage DNA cannot replicate.

The combined use of amber mutations and an amber suppressor produces a conditional mutant, which is a mutant that is expressed under some circumstances but not under others. Conditional mutants are especially useful for studying mutations in essential genes. Another kind of conditional mutation is a temperature sensitive mutation for which the mutant trait is exhibited at high temperature but not at low temperature. In a sense, auxotrophic mutations are also conditional because auxotrophic mutants can be grown in the presence of the required nutrient, but the mutants will not grow when the nutrient is not provided.