Volume 1

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    r//region will not grow at all on K12 and produces a different type of plaque —the r or rapid lysis phenotype—on E.coli B : wild type phage mutant rll phage Some types of mutant—termed leaky—show partial function in growing on strain K12, but when tested on E.coli B do not show the true wild type plaque. Acridines act as mutagens by causing addition or deletion of a base pair in the nucleotide sequence of DNA. Whereas mutants produced by mutagens which cause the substitution of one base for another are often leaky—typically about half are—those produced by acridine mutation usually lack the gene function completely. The difference between the two types of mutagen can be accounted for if the genetic code is read in non-overlapping triplets from a fixed point. In this case, misreading occurs after an addition or deletion because the phase of the message has been shifted by one nucleotide residue; the amino acid content of the corresponding protein is completely altered from this point on ; as figure 1.7 shows, this results in complete lack of function. (This would not happen if the phase were defined by the nucleotide sequence in a commaless code.) In a base substitution mutant, however, where only one or at most a few bases have been altered, only a few amino acids are changed in the protein. This may leave some residual protein activity, generating the leaky phenotype. Acridine mutants cannot be induced to revert by mutagens which cause base substitution, but can do so upon further mutation with acridine. If an acridine mutant is produced by, say, addition of a base, it should revert to wild type by deletion of that base. But in FC 0 , a mutant induced by acridine, this reversion can be achieved by producing a second mutation at another site in the same gene, instead of by reverting the original site of mutation. This second mutation is termed a suppressor (see later in this chapter). If FC 0 were formed by base addition, the original phase of reading could be restored by deleting another base somewhere nearby. Misreading then takes place only between the two sites of mutation; figure 1.7 shows that the parts of the gene outside them are read in the original phase. By forming double mutants through recombination between FC 0 and a phage carrying a second mutation, Crick et al. identified eight suppressors of the first mutation, all located within a short region of the same rll cistron. All these suppressors are themselves non-leaky mutants, so that their reversion to wild type can in turn be studied by the same procedure used to follow rever sion of FC 0 . They also revert to wild type when a double mutant is formed, the suppressors of the suppressors again comprising non-leaky r//mutants. If, E.coli B E.coli K12 normal (small fuzzy plaque) normal (small fuzzy plaque) large plaque (r type) no growth