In this article we discuss about the expression of mutations.
The expression of mutation can be observed only when there occurs a detectable and altered phenotype. If a mutation occurs from a most prevalent wild type to a mutant form, it is called forward mutation.
In forward mutation, a second mutation may take place resulting in appearance of a wild type organism again. This type of mutation is known as back mutation, reverse mutation or reversion. The reverse mutants are called reverants.
It is the phenomenon of reverting of a mutant organism to the original phenotype characteristics.
Reversion may occur spontaneously by induction or mutagens, for example:
The reversion frequency of an organism is a useful criterion for the identification of point mutation. However, if deletion has taken place, reversion will not occur. In bacteria, reversion events can be detected by measuring the ability of formation of bacterial colonies on solid growth medium.
For example, if 107 Leu– bacterial cells are plated on solid medium devoid of leucine about 60 colonies are formed. These colonies must be consisting of Leu+ bacteria which would have reverted spontaneously from Lac– to Lac+ cells.
Hence, the reversion frequency can be calculated as below:
No. of cells plated = 107
No. of colonies formed = 60
Reversion frequency = 60/107 = 6 x 10-6
This value of frequency is a feature of the reversion of point mutants. Since, the production of spontaneous reverants occurs randomly, the reversion of a double mutants requires two independent events. Therefore, the reversion frequency would be (6 x 10-6)2 = 3 x 10-11
II. Second Site Mutation (Suppressor Mutation):
Spontaneous reversion rarely results in a restoration of the wild type base sequences. Only 100 Leu– mutants found in a population of about 109 Leu+ cells and mutation causing mutants would be distributed over 100 different sites.
After the growth of these cells, in a second population, there may also be 100 mutants some having mutation at sites present in the first population but most of these having mutation on other sites.
In many population there will be about 500 mutation sites. If a reversion occurs at one particular site, it yields wild type base sequence. On average only 100/ 500 × 109 would be mutated at that site. Thus, the phenomenon of reversion into wild type phenotype by a second mutation in a different gene is called second site mutation or suppressor mutation.
The second site mutation overcomes the effect of first mutation. If the second mutation is within the same gene, the mutation is called a second site reversion or intragenic suppression. Although the reverant phenotypes are wild type, the original sequences of DNA will not be regained.
If the mutational changes occur in a second gene, it eliminates or suppresses a mutant phenotype which is called suppression or intergenic reversion. This type of suppression has been studied carefully with conditional mutation, which develops wild type phenotypes on certain conditions and produces mutant phenotype in other conditions.
The major class of this mutation is called suppressor sensitive mutation. It acts like wild type when a suppressor molecule is present. For example, a phage mutant can grow in one strain of bacteria but becomes unable to grow in other strain. Suppressor-sensitive mutations are of two types, non-sense (chain termination) mutation and missense (amino acid substitution) mutations.