Phage group

In a retrospective article,[2] Emory Ellis stated "Soon after Max Delbruck arrived at the Caltech Biology Division, intent on discovering how his background in physical sciences could be productively applied to biological problems, I showed him some step-growth curves. His first comment was ‘I don’t believe it.’" However as Ellis describes, Delbruck soon dispelled this initial reaction of disbelief by his own analysis of the phenomenon, and promptly joined in the work with enthusiasm, bringing to it his training in mathematics and physics, and intense interest in genetics. Their initial collaborative findings were published in 1939.[3]

The phage group started around 1940, after Delbrück and Luria had met at a physics conference. Delbrück and Salvador Luria began a series of collaborative experiments on the patterns of infection for different strains of bacteria and bacteriophage. They soon established the "mutual exclusion principle" that an individual bacterium can only be infected by one strain of phage. In 1943, their "fluctuation test", later dubbed the Luria–Delbrück experiment, showed that genetic mutations for phage resistance arise in the absence of selection, rather than being a response to selection.[4][5] The traditional wisdom among bacteriologists prior to 1943 was that bacteria had no chromosomes and no genes. The Luria–Delbrück experiment showed that bacteria, like other established model genetic organisms, have genes, and that these can spontaneously mutate to generate mutants that may then reproduce to form clonal lineages. That year, they also began working with Alfred Hershey, another phage experimenter.[6] (The three would share the 1969 Nobel Prize in Physiology or Medicine, "for work on the replication mechanism and genetics of viruses".)

Hershey,[7] described retrospectively the circumstances leading to the experiment using phage that he performed with his research assistant, Martha Chase, in 1952, later known as the Hershey–Chase experiment.[8] This experiment provided key evidence that DNA, as distinct from protein, is the genetic material of the phage and therefore the likely genetic material generally.

In 1946, Luria made a finding that was destined to open up a new insight on how the stability of DNA is achieved (see Luria,[5] pg. 96). What he discovered was that when, after UV irradiation, two or more "dead" phage entered the same bacterial cell, they often became alive again and produced normal live progeny.[9] This was the first example of reactivation of cells or organisms that had been damaged by radiation. He interpreted the reactivation, correctly, as a result of genetic recombination (see also homologous recombination). James Watson (future co-discover of the Watson–Crick structure of DNA in 1953, and winner of the Nobel Prize, 1962), was Luria’s first graduate student at the Indiana University. As his PhD thesis project, Watson showed that X-rayed phage can participate in genetic recombination and multiplicity reactivation.

As remembered by Luria (1984,[5] pg. 97) the discovery of reactivation of irradiated phage (referred to as "multiplicity reactivation") immediately started a flurry of activity in the study of repair of radiation damage within the early phage group (reviewed by Bernstein[10] in 1981). It turned out later that the repair of damaged phage by mutual help that Luria had discovered was only one special case of DNA repair. Cells of all types, not just, bacteria and their viruses, but all organisms studied, including humans, are now known to have complex biochemical processes for repairing DNA damages (see DNA repair). DNA repair processes are also now recognized as playing critical roles in protecting against aging, cancer, and infertility.

Jim Watson, in a retrospective article,[11] described his first experiences as a student with Luria in 1947. Apparently, according to Watson "…many students were afraid of Luria who had a reputation of being arrogant toward people who were wrong." However, as the fall term wore on, Watson "saw no evidence of the rumored inconsiderateness toward dimwits." Thus with no real reservations (except for occasional fear that he was not bright enough to move in his circle) he asked Luria whether he could do research under his direction in the spring term. Luria promptly said yes, and gave Watson the task of studying X-ray-induced multiplicity reactivation of phage as described above. The only other scientist in Luria’s lab at that time, with whom Watson shared a lab bench, was Renato Dulbecco (a future member of the phage group), who had recently arrived from Italy to do experiments on phage multiplicity reactivation. Later that semester (1948), Watson met, for the first time, Delbruck who was briefly visiting Luria. Watson[11] wrote "Almost from Delbruck’s first sentence, I knew I was not going to be disappointed. He did not beat around the bush and the intent of his words was always clear. But even more important to me was his youthful appearance and spirit." Watson noted that on this occasion, as on many subsequent occasions, Delbruck talked about Bohr (the physicist) and his belief that a complementarity principle, perhaps like that needed for understanding quantum mechanics, would be the key to the real understanding of biology.

In 1950, Renato Dulbecco now at Caltech with Delbrück, worked out a procedure for assaying animal virus particles by their formation of plaques on a sheet of cultured cells, just as phage form plaques on a lawn of bacterial cells. This procedure set the stage for Dulbecco to implement a comprehensive research program for quantitative studies on animal viruses to fathom their intracellular reproductive cycle. This work was recognized by award of the Nobel Prize in 1975.[12]

After the discovery of the structure of DNA in 1953, it was still unclear how DNA replicated. The favored model at the time was semi-conservative replication, but experimental proof was needed. The Meselson–Stahl experiment,[13] performed by Matthew Meselson and Franklin Stahl in 1958, was the key experiment that provided convincing evidence of semi-conservative replication, the mechanism now known to be correct. Meselson and Stahl described the circumstances leading to this key experiment.[14] It has since been described as the "Most Beautiful Experiment in Biology."[15] Its beauty is tied to the simplicity of the result, although the route that led to the experiment was far from simple.

As described in a retrospective article,[16] Seymour Benzer joined Delbrück's phage group at Caltech in 1949 as a postdoctoral fellow. There he shared a lab room with Jean Weigle where they did collaborative experiments on phage T4. Upon leaving Caltech, Benzer continued experiments on phage T4 at the Pasteur Institute in Paris and then Purdue University where he developed a system for studying the fine structure of the gene using mutants defective in the rIIA and rIIB genes.[17][18] These genetic experiments, involving crosses of rII mutants, led to the finding of a unique linear order of mutational sites within the genes. This result provided strong evidence for the key idea that the gene has a linear structure equivalent to a length of DNA with many sites that can independently mutate.

In 1952, Salvador Luria had discovered the phenomenon of "restriction-modification" (the modification of phage growing within an infected bacterium, so that upon their release and re-infection of a related bacterium the phage’s growth is restricted) (described by Luria,[5] pgs. 45 and 99). Weigle, working with Giuseppe Bertani and Werner Arber, soon clarified the basis for this phenomenon. They showed that restriction was actually due to attack by specific bacterial enzymes on the modified phage’s DNA. This work led to the discovery of the class of enzymes now known as "restriction enzymes." These enzymes allowed controlled manipulation of DNA in the laboratory, thus providing the foundation for the development of genetic engineering.

Weigle also demonstrated the inducible nature of DNA damage-response genes in bacteria, a phenomenon that has come to be known as the SOS response. This response includes DNA damage-inducible mutagenesis (termed Weigle mutagenesis in his honor) and inducible repair following DNA damage (termed Weigle reactivation).

In 1961, Sydney Brenner, an early member of the phage group, collaborated with Francis Crick, Leslie Barnett and Richard Watts-Tobin at the Cavendish Laboratory in Cambridge to perform genetic experiments that demonstrated the basic nature of the genetic code for proteins.[19] These experiments, carried out with mutants of the rIIB gene of phage T4, showed, that for a gene that encodes a protein, three sequential bases of the gene’s DNA specify each successive amino acid of the protein. Thus the genetic code is a triplet code, where each triplet (called a codon) specifies a particular amino acid. They also obtained evidence that the codons do not overlap with each other in the DNA sequence encoding a protein, and that such a sequence is read from a fixed starting point.

Gunther Stent joined the phage group in 1948 after taking their phage course at Cold Spring Harbor, New York. The ongoing informal discussions among these workers on the progress of their research led to a book by Stent entitled Molecular Biology of Bacterial Viruses (dedicated to Max Delbrück)[20] which was a lucid account of the accomplishments in this emerging field up to 1963. Later, in his memoirs, Stent (1998) described some of the activities and personal interactions that illustrated the unique intellectual spirit of the phage group during its early crucial years (1948-1950).[21]

Delbrück, through his charm and enthusiasm, brought many biologists (and physicists) into phage research in the early 1940s (see:Charles Steinberg). [22] In 1944, Delbrück promoted the "Phage Treaty", a call for phage researchers to focus on a limited number of phage and bacterial strains, with standardized experimental conditions. This helped to make research from different laboratories more easily comparable and replicable, helping to unify the field of bacterial genetics.[23]