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T4 rII system
The T4 ''r''II system is an experimental system developed in the 1950s by Seymour Benzer for studying the substructure of the gene. The experimental system is based on genetic crosses of different mutant strains of bacteriophage T4, a virus that infects the bacteria ''E. coli''.
==Origin==
One type of mutation in the T4 bacteriophage identified by researchers in Phage genetics by the 1950s was known as ''r'' (for ''rapid''), which caused the phage to destroy bacteria more quickly than normal. These could be spotted easily because they would produce larger plaques rather than the smaller plaques characteristic of the wild type virus. Through genetic mapping, the researchers had identified specific regions in the T4 chromosome, called the ''r''I, ''r''II, and ''r''III loci, associated with the ''r'' mutants. In 1952, while performing experiments with ''r''II mutants, Seymour Benzer found a strain that did not behave normally. By 1953, after the publication of Watson and Crick's proposed structure of DNA, Benzer hit on the idea that the apparently defective ''r'' mutants might have been the result of crossing two different ''r''II mutants, each of which had part of the ''r''II gene intact, so that the hybrid strain did not exhibit the ''r'' phenotype at all because it combined the intact parts of the ''r''II gene.〔Weiner, p. 52.〕
From there, Benzer saw that it would be possible to generate many independent ''r'' mutants, and by measuring the recombination frequency between different ''r'' strains, he could map the substructure of a single gene. Although the chance of successful recombination for any given virus was very small, a single petri dish could be the basis for millions of trials at once. They could be screened easily by using a specific strain of ''E. coli'', known as K12 (λ), that was susceptible to wild type T4 but not to ''r'' mutants.〔Jayaraman, p. 903〕
Benzer's concept was quite controversial within classical genetic thought, in which each gene is treated as a singular point along a chromosome, not a divisible stretch of nucleic acids (as implied by the work of Watson and Crick). Initially, Max Delbrück—a respected phage geneticist and leader of the so-called phage group of which Benzer was a part—found Benzer's idea outrageous.〔Weiner, p. 53〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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