Phage played a major role in the development of the field of molecular biology. Use of phage as model systems for understanding general biological problems was stimulated by an informal collection of researchers called the "Phage group". After World War II, many physicists were attracted to biology and the relativity simplicity of phage seemed like an ideal place to begin to dissect complex biological questions like what is the nature of a gene, how do mutations affect genes, how do mutations arise, how do genes replicate, and how are genes expressed. Many of the newcomers to the new field of molecular genetics were first initiated to phage research in a summer "phage course" taught by Salvador Luria and Max Delbruck at Cold Spring Harbor Laboratory.

A few important discoveries that relied upon phage research include:

Central dogma:

• DNA is the genetic material - phage T2 (Hershey and Chase, 1952)
• Gene and protein sequences are colinear - phage T4 (Brenner, 1964)
• mRNA is an intermediate in translating genetic information from DNA to protein (Volkin and Astrachan, 1956 and 1957)
• The genetic code is determined by three, nonoverlapping nucleotides - phage T4 (Crick et al, 1961)
• DNA replication is initiated from a complex of proteins and a nucleotide primer - phage phiX174 (Goulian and Kornberg, 1967)
• Lagging strand DNA synthesis is discontinuous - phage T4 (Okazaki and Okazaki, 1969)
• DNA ligase catalyzes the covalent joining of DNA molecules - phage lambda (Gellert, 1967)
• Protein chaperonins facilitate protein folding - phage lambda (Georgopoulos et al., 1973)

Molecular genetics:

• Deletion mapping provides a simple way to determine the fine-structure of a gene - phage T4 (Benzer, 1957)
• Recombination occurs at the nucleotide level - phage T4 (Benzer, 1957)
• Insertion of DNA can occur by recombination with a circular DNA molecule - phage lambda (Campbell, 1959)
• Mutations occur randomly and are fixed in a population by genetic selection - phage T1 (Luria and Delbruck, 1943)
• Different mutagens cause a distinct pattern of mutations -- i.e. different mutagens have unique hotspots - phage T4 (Benzer, 1957)
• Temperature sensitive and amber mutations have conditional phenotypes that can be used to study essential genes - phages T4 and lambda (Edgar and Lielausis, 1964; Campbell, 1961)
• Nonsense codons are translation stop signals (Benzer and Champe, 1962)
• Restriction-modification systems can determine the fate of introduced DNA - phages lambda and P1 (Luria and Human, 1952; Dussoix and Arber, 1962)
• Phage can alter the physiological properties of the host cell via lysogenic conversion - Corynebacterium diphtheriae phages (Freeman, 1951)
• Phage can transfer chromosomal DNA between bacteria via transduction - phage P22 (Zinder and Lederberg, 1952)

Genetic regulation:

• Transcriptional regulation can occur by a protein binding to DNA sites - phage lambda (Jacob and Monod, 1961; Ptashne, 1967)
• Transcriptional regulation can occur by a inhibition of repression by an anti-repressor - phage P22 (Levine et al., 1975; Botstein et al., 1975)
• Regulation of gene expression can occur by anti-termination of transcription - phage lambda (Roberts, 1969)
• Regulation of gene expression can occur by mRNA degradation - phage lambda (Guarneros and Galindo, 1979)

Ecology:

• Phage can determine the population dynamics of bacteria in nature (Bergh et al., 1989)

Methods:

• Plaque assays to quantitate viable viruses (D'Herelle, 1917)
• Gene cloning - phage lambda (Berg, 1972; Lobban and Kaiser, 1973)
• SDS polyacrylamide gel electrophoresis of proteins - phages T4 and T7 (Laemmli, 1971; Studier, 1973)
• Linear recombination using donor DNA fragments with very short regions of homology
• Phage display
• Overexpression of cloned genes using phage RNA polymerase

[If you think of other important examples that should be added, please let me know.]

REFERENCES:

• Brock, T. 1990. The emergence of bacterial genetics. Cold Spring Harbor Laboratory Press, NY.
• Cairns, J., G. Stent, and J. Watson. 1966. Phage and the origins of molecular biology. Cold Spring Harbor Laboratory of Quantitative Biology, NY.
• Stent, G. 1963. Molecular Biology of Bacterial Viruses. W. H. Freeman and Co., San Francisco.
• Wommack, K., R. Colwell. 2000. Virioplankton: viruses in aquatic ecosystems. Microbiol. Mol. Biol. Rev. 64: 69-114.

• ASM Division M web site - http://www.asmusa.org/division/m/blurbs/Secrets.html