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A Biochemical Mechanism for Nonrandom Mutations and Evolution
A multitude of random mechanisms result in hypermutation under conditions of environmental stress and clearly contribute to the variability essential to evolution. However, since most mutations are deleterious, random mechanisms that increase mutation rates also result in genomewide DNA damage. Among microorganisms, from phage to fungi, the overall mutation rate per genome is remarkably constant (within 2.5-fold), presumably reflecting an obligatory, delicate balance between the need for variation and the need to avoid general genetic damage (24, 45, 57). Thus, mutator strains are not selected in nature but remain at 1 to 2% of the population (35, 52); under certain adverse conditions, they flourish for short periods but are then selected against, apparently because of widespread deleterious effects intrinsic to genomewide hypermutation. In contrast, hypermutation that is the consequence of starvation-induced derepression and transcriptional activation represents a very rapid and specific response to each adverse circumstance. The extent to which normal background mutations in nature are due to derepression mechanisms is difficult to estimate, but the location of most C-to-T transitions on the nontranscribed strand suggest that it may be significant. Regardless, a mechanism that limits an increase in mutation rates to genes that must mutate in order to overcome prevailing conditions of stress would surely be beneficial and therefore selected during evolution.