Directed mutagenesis
Encyclopedia
Directed mutagenesis, also known as directed mutation, is a hypothesis
proposing that organisms can respond to environmental stresses through directing mutations to certain genes
or areas of the genome
.
The hypothesis was first proposed in 1988 by John Cairns
, of Harvard University
, who was studying Escherichia coli
that lacked the ability to metabolize lactose
. He grew these bacteria in media
in which lactose was the only source of energy
. In doing so, he found that the rate at which the bacteria evolved the ability to metabolize lactose was many orders of magnitude higher than would be expected if the mutations were truly random. This inspired him to propose that the mutations that had occurred had been directed at those genes involved in lactose utilization.
Later support for this hypothesis came from Susan Rosenberg, then at the University of Alberta
, who found that an enzyme involved in DNA recombinational repair
, recBCD
, was necessary for the directed mutagenesis observed by Cairns and colleagues in 1989.
The directed mutagenesis hypothesis was challenged in 2002, when John Roth and colleagues showed that the phenomenon was due to general hypermutability due to selected gene amplification, and was thus a "standard Darwinian process." Later research published in 2006 by Jeffrey D. Stumpf, Anthony R. Poteete, and Patricia L. Foster, however, concluded that amplification could not account for the adaptive mutation and that "mutants that appear during the first few days of lactose selection are true revertants that arise in a single step".
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
proposing that organisms can respond to environmental stresses through directing mutations to certain genes
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
or areas of the genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
.
The hypothesis was first proposed in 1988 by John Cairns
John Cairns (biochemist)
John Forster Cairns FRS is a British physician and molecular biologist who made significant contributions to molecular genetics, cancer research, and public health....
, of Harvard University
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
, who was studying Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
that lacked the ability to metabolize lactose
Lactose
Lactose is a disaccharide sugar that is found most notably in milk and is formed from galactose and glucose. Lactose makes up around 2~8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from or , the Latin word for milk,...
. He grew these bacteria in media
Growth medium
A growth medium or culture medium is a liquid or gel designed to support the growth of microorganisms or cells, or small plants like the moss Physcomitrella patens.There are different types of media for growing different types of cells....
in which lactose was the only source of energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
. In doing so, he found that the rate at which the bacteria evolved the ability to metabolize lactose was many orders of magnitude higher than would be expected if the mutations were truly random. This inspired him to propose that the mutations that had occurred had been directed at those genes involved in lactose utilization.
Later support for this hypothesis came from Susan Rosenberg, then at the University of Alberta
University of Alberta
The University of Alberta is a public research university located in Edmonton, Alberta, Canada. Founded in 1908 by Alexander Cameron Rutherford, the first premier of Alberta and Henry Marshall Tory, its first president, it is widely recognized as one of the best universities in Canada...
, who found that an enzyme involved in DNA recombinational repair
DNA repair
DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...
, recBCD
RecBCD
RecBCD, also known as Exonuclease V, is an enzyme of the E. coli bacterium that initiates recombinational repair from potentially lethal double strand breaks in DNA which may result from ionizing radiation, replication errors, endonucleases, oxidative damage, and a host of other factors...
, was necessary for the directed mutagenesis observed by Cairns and colleagues in 1989.
The directed mutagenesis hypothesis was challenged in 2002, when John Roth and colleagues showed that the phenomenon was due to general hypermutability due to selected gene amplification, and was thus a "standard Darwinian process." Later research published in 2006 by Jeffrey D. Stumpf, Anthony R. Poteete, and Patricia L. Foster, however, concluded that amplification could not account for the adaptive mutation and that "mutants that appear during the first few days of lactose selection are true revertants that arise in a single step".
External links
- "Accurate prediction of the stability and activity effects of site-directed mutagenesis on a protein core" - Nature
- "Rapid and efficient site-directed mutagenesis by the single-tube megaprimer PCR method" - Nature
- "NOBEL WINNER IN CHEMISTRY" - Miami Herald
- "BASF seeks GM alternative" - RSC
- "Tinkering with enzymes: what are we learning?" - Science