Intragenic vs. intergenic suppression
Intragenic suppression Intragenic suppression results from suppressor mutations that occur in the same
gene as the original mutation. In a classic study,
Francis Crick (et al.) used intragenic suppression to study the fundamental nature of the
genetic code. From this study it was shown that genes are expressed as non-overlapping triplets (
codons). These strains are likely not subject to losing these compensatory mutations and which would greatly decrease the fitness in the strain resulting in the intermediate strains. These intermediate strains are subjected to bottlenecking and thus making it difficult for the alleles to be reverted prior to Intergenic suppressions. Consequently, when drugs are halted it can be seen that these mutations are likely to persist in the population. Suppressor mutations also occur in genes that code for virus structural proteins. To create a viable
phage T4 virus (see image), a balance of structural components is required. An
amber mutant of phage T4 contains a mutation that changes a codon for an amino acid in a protein to the nonsense stop codon TAG (see
stop codon and
nonsense mutation). If, upon infection, an amber mutant defective in a gene encoding a needed structural component of phage T4 is weakly suppressed (in an
E. coli host containing a specific altered
tRNA – see
nonsense suppressor), it will produce a reduced number of the needed structural component. As a consequence few if any viable phage are formed. However, it was found that viable phage could sometimes be produced in the host with the weak nonsense suppressor if a second amber mutation in a gene that encodes another structural protein is also present in the phage genome. It was found that the reason the second amber mutation could suppress the first one is that the two numerically reduced structural proteins would now be in balance. For instance, if the first amber mutation caused a reduction of tail fibers to one tenth the normal level, most phage particles produced would have insufficient tail fibers to be infective. However, if a second amber mutation is defective in a base plate component and causes one tenth the number of base plates to be made, this may restore the balance of tail fibers and base plates, and thus allow infective phage to be produced. ==Revertant==