Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 153

The phenotypes caused by mutation can revert in one of two ways: by change of the mutation to the wild-type sequence or by acquisition of a mutation at a second site, either in the same gene or a different gene. Phenotypic reversion caused by second-site mutation is known as suppression, or pseudoreversion, to distinguish it from reversion at the original site of mutation. Reversion has been studied since the beginnings of classical genetic analysis. In the modern era of genetics, cloning and sequencing techniques can be used to demonstrate suppression and to identify the nature of the suppressor mutation (see below). The identification of suppressor mutations is a powerful tool for studying protein-protein and protein-nucleic acid interactions. Some mutations complement changes made at several sites, whereas allele-specific suppressor mutations complement only a specific change. The allele specificity of second-site mutations provides evidence for physical interactions among proteins and nucleic acids.

Phenotypic revertants can be isolated either by propagating the mutant virus under restrictive conditions or, in the case of mutants exhibiting phenotypes (e.g., small plaques), by searching for wild-type properties. Chemical mutagenesis may be required to produce revertants of DNA viruses but is not necessary for RNA viruses, which spawn mutants at a higher frequency. Nucleotide sequence analysis is then used to determine if the original mutation is still present in the genome of the revertant. The presence of the original mutation indicates that reversion has occurred by second-site mutation. The suppressor mutation is identified by nucleotide sequence analysis. The final step is introduction of the suspected suppressor mutation into the genome of the original mutant virus to confirm its effect. Several specific examples of suppressor analysis are provided below.