Crick, Brenner et al. experiment

The Crick, Brenner, Barnett, Watts-Tobin experiment of 1961 was a scientific experiment performed in 1961 by Francis Crick, Sydney Brenner, Leslie Barnett and R.J. Watts-Tobin. They demonstrated that three bases of DNA code for one amino acid in the genetic code. The experiment elucidated the nature of gene expression and frame-shift mutations.

In the experiment, proflavin-induced mutations of the T4 bacteriophage gene, rIIB, were isolated. Proflavin causes mutations by inserting itself between DNA bases, typically resulting in insertion or deletion of a single base pair.

The mutants produced by Crick and Brenner could not produce functional rIIB protein because the insertion or deletion of a single nucleotide caused a frameshift mutation. Mutants with two or four nucleotides inserted or deleted were also nonfunctional. However, the mutant strains could be made functional again by using proflavin to insert or delete a total of three nucleotides. This proved that the genetic code uses a codon of three DNA bases that corresponds to an amino acid.

Sydney Brenner, in his autobiographical book (with other contributors), describes the origin of this experiment on pages 91 to 96. He wrote: “It was possible to get a mutation in one place which would alter a protein, and then you could get another mutation elsewhere [in the same gene] which corrected the protein. It’s called an internal suppressor.” “Francis [Crick] started to play around in the lab with [rII] mutants. He started with a few of the base analogue mutants and these did not give him much joy. But where he found innumerable suppressors was with the acridine mutants [these were addition/deletion mutants]… You could start with a mutant which was arbitrarily called ‘plus’ because it gained a base. Then all the suppressors of it would be ‘minuses’ because when you add a plus and a minus you came back to zero….If you put [recombined] three [plus] mutants together [or three minus mutants together] they came back to normal! And if you mixed four or five they were still mutant. We went as far as six to make it normal and after that it got a bit boring.” “This concept of a phase shift, or a ‘frameshift’ [in the genetic code of an rII gene] as we later called it, was so foreign to people in genetics that we had endless problems trying to explain this work.”

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