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Mutator and Antimutator Mutations
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Whereas intergenic suppressors represent mutations that restore the normal phenotype, mostly through mutation of transfer RNA loci, mutator and antimutator mutations cause an increase or decrease in the overall mutation rate of the cell. They are frequently mutations of DNA polymerase, which, as you remember, not only polymerizes DNA nucleotides 5 3 complementary to the template strand, but also checks to be sure that the correct base was put in (they proofread). If, in the proofreading process, the polymerase discovers an error, it can correct this error with its 3 5 exonuclease activity. Mutator and antimutator mutations sometimes involve changes in the polymerase s proofreading ability (exonuclease activity). Phage T4 has its own DNA polymerase with known mutator and antimutator mutants. Mutator mutants are very poor proofreaders (they have low exonuclease-topolymerase ratios), and thus they introduce mutations throughout the phage genome. Antimutator mutants, however, have exceptionally ef cient proofreading ability (high exonuclease-to-polymerase ratios) and, therefore, result in a very low mutation rate for the whole genome (box 12.3).
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Figure 12.26 Frameshift and nonsense suppression by mutant
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transfer RNAs. In (a), a thymine has been inserted into DNA, resulting in a frameshift. However, a transfer RNA with four bases in the codon region reads the inserted base as part of the previous codon in the messenger RNA. The frameshift thus does not occur. In (b), an amber mutation (UAG), which normally results in chain termination, is read as tyrosine by a mutant tyrosine transfer RNA that has the anticodon sequence complementary to the amber codon.
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Tamarin: Principles of Genetics, Seventh Edition
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III. Molecular Genetics
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12. DNA: Its Mutation, Repair, and Recombination
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BOX 12.3
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lthough we discuss evolution in detail at the end of the book, note here that we view mutation as a random process, not one that occurs because a cell needs a particular mutation. For example, Luria and Delbr ck s work showed that the mutation in E. coli for resistance to phage T1 occurred randomly before exposure to the phage, not because the cells would bene t from the mutation. Because of the entrenched dogma that mutations occur through random processes, the scienti c community was startled when, in 1988, John Cairns a highly respected senior scientist and colleagues reported a new observation adaptive, or directed, mutation, occurring when the cell needed it. Their system was the lacZ gene in E. coli. Cells that could not use lactose as an energy source (lacZ ) were plated on a medium in which lactose was the sole energy source. Some mutants already there of course produced colonies (lacZ ). The expectation was that no new mutations would occur over time because the lacZ cells would have either died or stopped metabolizing. Unexpectedly, Cairns and colleagues found that more and more colonies appeared over time, coming from cells that had mutated to lacZ . As a control, they looked for revertants of other genes not involved with lactose metabolism. These mutations did not occur in a directed manner. Scientists were extremely skeptical of this work for two reasons. First,
Experimental Methods
Adaptive Mutation
it seemed to y in the face of our common understanding of the mutational process. Second, there were no obvious explanations for how this could occur. Numerous articles were published refuting the notion of adaptive mutations and suggesting other explanations for the results. These explanations included artifacts of miscounting cells to mutants that were extremely slow growing, but there all the time. In the past several years, other scientists have found at least a half dozen similar results in other organisms and other genes. The debate is ongoing, but work published in mid1994 seems to have recast it into the realm of methods of mutagenesis rather than non-Darwinian processes. Several scientists found that the directed mutations seem to be of a certain type, mainly single nucleotide deletions within runs of the same nucleotide: for example, a deletion of a C in a CCCC sequence. This type of error happens during DNA replication and could be the result of a repair de ciency. That is, under extreme duress, the cells may be going into a hypermutational mode, or
selection may favor hypermutable genotypes in which repair mechanisms are shut down in order to intentionally create lots of errors in the DNA. Any errors that do not alleviate the problem result in cell death, a death that was inevitable anyway; however, some errors will correct the problem (lacZ to lacZ ). Those cells will survive. One recent study indicated that a subpopulation of about 0.06% of the population was hypermutable, with a mutation rate about 200 times that of the normal cells. That group of cells could account for the adaptive mutations. Other scientists found that when the locus of importance was on a plasmid, the adaptive mutation could occur by increased replication of the plasmid. (Currently, the favored term is adaptive mutation, rather than directed mutation, indicating that the mutations are useful to the cell, not that some unknown process directs them.) This controversy, although not necessarily resolved, has actually brought out the best in the scienti c method: A skeptical scienti c community tried its best to refute an unreasonable observation. Other scientists then repeated the observation and extended it, making it more worthy of further study. Finally, further work has given us reasonable mechanisms that not only require no rejection of the original concept of random mutation, but actually give us hypotheses to test further.
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