Saturday, December 1, 2007

This Week's Citation Classic

This week's citation classic honors Seymour Benzer who passed away yesterday.

Benzer S. 1955. Fine structure of a genetic region in bacteriophage. PNAS 41(6):344-54.

Benzer began his career as a physicist, but was inspired to switch to molecular biology by Schrodinger's book What is Life and by Max Delbruck. He took a leave of absence from his position at Purdue to pursue research on the structure of the gene, and he never looked back. Biology, bacteriophages in particular, captivated him.

In his classic 1955 PNAS paper, Benzer used the bacteriophage T4 to map the r gene.

Benzer describes his experiments:

I plated some of these r mutants on two different strains of E. coli bacteria. And I had two different strains of K-12 phage, one that was lysogenic and had Lwoff’s lambda phage in it, and one that didn’t. What happened first was that when I plated these r mutants on the plain K-12 strain, instead of making the big plaques they made the little plaques. So they were showing lysis inhibition on that strain. Then, when I plated them on the strain that had the lambda, I got zero plaques. And having been alerted by reading Pontecorvo’s article, I immediately, really instantly, realized…. Well, at first I thought I made a mistake. I thought I had forgot to put the phage on there. Dummkopf, do it again! I did it again and saw the same phenomenon.

Benzer's key insight was that he had stumbled upon a system where he could do genetic mapping by using recombination between r mutants to map the r gene much in the way that Alfred Sturtevant used recombination to map Drosophila. The trouble with using recombination to map genes is that the closer two genes are together, the less likely they will recombine. Thus mapping a single gene was thought to be impossible.

But not so for phage. So many offspring (~100 million) are produced that the odds are that recombination can occur even among two adjacent nucleotides.

Benzer describes the moment:

So I immediately realized—a eureka moment, and they’re all too rare—that this was a system in which I could do very fine genetic mapping. I could take two r mutants, cross them with each other, and take the progeny and put them on this K-12 lambda strain. The r mutants themselves would produce no plaque, but if in any of the progeny there was a crossing-over between these two different mutations, such as to produce a wild-type recombinant that had neither mutation, that would produce a plaque. And that you could put 100 million plaques on one plate. So a quick calculation told me that that was enough, knowing the number of nucleotides in the DNA of the bacteriophage. This was about 1954 or ’55—after the Watson-Crick discovery. So, based on the number of nucleotides in the DNA and the phage, I would have enough resolving power to separate the rII mutations, even if they were just one nucleotide apart.

When r mutants are plated on lysogenic K-12 E. coli, only those that have undergone recombination within the r gene to form wild-type phage will form plaques. The number of plaques on the bacterial lawn indicates how far apart the two nucleotide mutations were on the gene. Thus, Benzer had the tool of exceptionally high resolution; he could to map the r gene down to the nucleotide.

Benzer's results showed conclusively that genes were not indivisible as commonly thought.

Benzer later went on to dissect the nervous system of Drosophila in a third highly productive career.

Even at 86 years of age, Benzer still ran a laboratory at CalTech. A very entertaining oral history interview series with Benzer is available here. Benzer's CV should give us all pause; he was a tremendously accomplished man.

Updates: Larry Moran points out this paper: Adventures in the rII Region and other tributes to Benzer in the blogosphere.

1 comment:

  1. Wow... thank you for your breakdown.. I have to read this for my method and logic class. Context really helps.