Adaptive mutation of a lacZ amber allele

Stress responses and genetic variation in bacteria

Under stressful conditions mechanisms that increase genetic variation can bestow a selective advantage. Bacteria have several stress responses that provide ways in which mutation rates can be increased. These include the SOS response, the general stress response, the heat-shock response, and the stringent response, all of which impact the regulation of error-prone polymerases. Adaptive mutation appears to be process by which cells can respond to selective pressure specifically by producing mutations. In Escherichia coli strain FC40 adaptive mutation involves the following inducible components: (i) a recombination pathway that generates mutations; (ii) a DNA polymerase that synthesizes error-containing DNA; and (iii) stress responses that regulate cellular processes. In addition, a subpopulation of cells enters into a state of hypermutation, giving rise to about 10% of the single mutants and virtually all of the mutants with multiple mutations. These bacterial responses have implications for the development of cancer and other genetic disorders in higher organisms.

Catastrophe and what to do about it if you are a bacterium: the importance of frameshift mutants

Key problems that bacteria have historically faced are the challenges of the lack of essential nutrients and the presence of antibiotics produced naturally, but there are many other challenges. It appears that for many of these challenges the bacteria have mechanisms encoded in their genomes that are not usually functioning, but may be "turned on" when needed, even if the need only occurs once in hundreds of thousands of generations. Such mechanisms at other times somehow need to be "turned off" because they may cause a slight disadvantage, or even a grave disadvantage to the cell compared with wild-type cells during the time the population is not being challenged. On the other hand, a gene cannot simply be discarded because it might be needed again. How do microorganisms solve the problem of responding to challenges that only occur rarely? I suggest that in most cases, the mutation must occur by the existence of a readily reversible mutation. The mutation in likely the result of a frameshift mutation that caused the response and later another frameshift occurs to return the genome to its original state.

Error-prone polymerase, DNA polymerase IV, is responsible for transient hypermutation during adaptive mutation in Escherichia coli

The frequencies of nonselected mutations among adaptive Lac(+) revertants of Escherichia coli strains with and without the error-prone DNA polymerase IV (Pol IV) were compared. This frequency was more than sevenfold lower in the Pol IV-defective strain than in the wild-type strain. Thus, the mutations that occur during hypermutation are due to Pol IV.

Mechanisms of stationary phase mutation: a decade of adaptive mutation

A decade of research on adaptive mutation has revealed a plethora of mutagenic mechanisms that may be important in evolution. The DNA synthesis associated with recombination could be an important source of spontaneous mutation in cells that are not proliferating. The movement of insertion elements can be responsive to environmental conditions. Insertion elements not only activate and inactivate genes, they also provide sequence homology that allows large-scale genomic rearrangements. Some conjugative plasmids can recombine with their host's chromosome, and may acquire chromosomal genes that could then spread through the population and even to other species. Finally, a subpopulation of transient hypermutators could be a source of multiple variant alleles, providing a mechanism for rapid evolution under adverse conditions.