Hypermutation in bacteria and other cellular systems.

A temporary state of hypermutation can in principle arise through an increase in the rate of polymerase errors (which may or may not be triggered by template damage) and/or through abrogation of fidelity mechanisms such as proofreading and mismatch correction. In bacteria there are numerous examples of transient mutator states, often occurring as a consequence of stress. They may be targeted to certain regions of the DNA, for example by transcription or by recombination. The initial errors are made by various DNA polymerases which vary in their error-proneness: several are inducible and are under the control of the SOS system. There are several structurally related polymerases in mammals that have recently come to light and that have unusual properties, such as the ability to carry out 'accurate' translesion synthesis opposite sites of template damage or the possession of exceedingly high misincorporation rates. In bacteria the initial errors may be genuinely spontaneous polymerase errors or they may be triggered by damage to the template strand, for example as a result of attack by active oxidative species such as singlet oxygen. In mammalian cells, hypermutable states persisting for many generations have been shown to be induced by various agents, not all of them DNA damaging agents. A hypermutable state induced by ionizing radiation in male germ cells in the mouse results in a high rate of sequence errors in certain unstable minisatellite loci; the mechanism is unclear but believed to be associated with recombination events.