Phage lambda

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Bacteriophage λ is the archetype of the lambdoid, or lambda-like phage, a subclass of the siphoviridae. λ is a temperate phage; it can grow either lytically or lysogenically. This feature has made λ especially important to the field of molecular biology. Discoveries based on λ include the physical nature of and site-specific recombination, DNA ligase, SDS-PAGE, cloning, and retroregulation.[1][2][3][4][5][6] The λ strain used most often in labs, although called wt, is λPaPa, named after the parental strains from Pasedena California and the Institut Pasteur in Paris. [7]

Morphology

The λ phage has an icosahedral head about 50 nm in diameter, and a flexible, noncontractile tail about 150nm long.[8][7] The head is formed from the proteins E, D, B, W, FII, B*, X1, and X2. [9] The head is the capsid, and contains a single double-stranded DNA molecule about 48 kb in length; by weight the genome is about 32x106 Da and comprises about half of the phage's weight.[8] The tail consists of a 135nm hollow tube finished with a 15nm conical cap.[8] The inside diameter of the tail is about 3nm, with an outside diameter of 9-18nm due to various "knobs" that make the tail appear rough.[8] The wt λ does not have tail fibers, although these may be present on certain strains.[7]

Genome Organization

The DNA is linear when packaged, and recircularizes using host ligase at the cos (cohesive end site) once injected into the host.[8] [9]. The information strand, rightmost (Rz) to leftmost (nu1), contains regions for lysis, late control, a non-essential region, DNA replication, repression (cII,cro, cI), antitermination (cIII, recombination, excision & integration, another non-essential region, then tail genes and head genes.[8] The att site (attachment) is between the recombination and the rightmost non-essential regions.[8] The cos region consists of the cosQ, cosN, and cosB subsites, and is flanked by nu1.

Life Cycle

Lytic Cycle

The lytic cycle produces progeny phage particles and results in the death of the host bacteria.

Adsorbtion and Injection

The tip of the phage tail (J protein) attaches to the host cell protein lamB, which is displayed on the outer membrane.[8] The dsDNA is injected through the tail as a linear genome, when the phage tail interacts with the inner membrane protein ptsM and recircularizes once inside the host due to complementary 12nt long sticky ends. [8]

Early Gene Expression

Transcripton occurs almost immediately from the pR (for promoter right), producing the N gene product. [10]


The cro gene produces an immediate-early gene product.[11] Gam is an early gene product, and CII and CIII are delayed-early gene products. [12][11]

Middle Gene Expression

The cro repressor truns off the early genes.[8]


DNA Replication

Replication directly after injection proceeds via θ (theta) replication after host gyrase supercoils the λ DNA. [9] Replication proceeds via rolling circle replication when entering the lytic cycle, aided by phage-encoded endonuclease which cuts the (+) strand.[9]

Late Gene Expression

Packaging

The DNA replicated by rolling circle remains as concatomers until terminase cleaves the cos site and one end of the DNA is packed into the capsid.[9] Proteins B, C, and Nu3 form the capsid precursors, which are aided by host groEL and groES to combine with E to form an immature capsid. [9] The proteins E and C combined and are reformed into X1 and X2, which make a mature capsid once B* is added (from B) and Nu3 is degraded. [9]

D protein is added to the capsid and standardizes the size of the head , then the λ DNA is inserted until the terminase cuts the DNA at the next cos site and W and FII proteins stabilize the capsid.[13] [9]

Tails are formed seperately from the far end of the structure in the proteins in the order J, I, L, K, H, G, M, V, U, H, and Z and attach spontaneously to mature capsids by proteins W and FII.[9]

Lysis

Proteins R and S are used to degrade the bacterial membranes, with S acting on the inner and R on the outer membrane. [9] About 100 phage are released per host, and infection to release takes about 35 minutes (40 min at 37˚C). [9][8]

Lysogenic State

The lysogenic lifecycle is the result of integration of the λ DNA into the host genome, and does not result in the death of the host cell. The λ DNA/host DNA unit is termed a prophage, and decendants of the prophage are capable of producing phage when the λ DNA excises from the host DNA. [14] The λ DNA is integrated at the attP (for attachment phage) and the attB (for attachment bacteria) loci.[11]

Lysis-Lysogeny Switch

Prophage Induction

See also

References

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  1. ↑ MESELSON, M & WEIGLE, JJ (1961) Chromosome brekage accompanying genetic recombination in bacteriophage. Proc. Natl. Acad. Sci. U.S.A. 47 857-68 PubMed
  2. ↑ Gellert, M (1967) Formation of covalent circles of lambda DNA by E. coli extracts. Proc. Natl. Acad. Sci. U.S.A. 57 148-55 PubMed
  3. ↑ Signer, ER & Weil, J (1968) Site-specific recombination in bacteriophage lambda. Cold Spring Harb. Symp. Quant. Biol. 33 715-9 PubMed
  4. ↑ Weber, K & Osborn, M (1969) The reliability of molecular weight determinations by dodecyl sulfate-polyacrylamide gel electrophoresis. J. Biol. Chem. 244 4406-12 PubMed
  5. ↑ Lobban, PE & Kaiser, AD (1973) Enzymatic end-to end joining of DNA molecules. J. Mol. Biol. 78 453-71 PubMed
  6. ↑ Guarneros, G & Galindo, JM (1979) The regulation of integrative recombination by the b2 region and the cII gene of bacteriophage lambda. Virology 95 119-26 PubMed
  7. ↑ 7.0 7.1 7.2 Hendrix, RW & Duda, RL (1992) Bacteriophage lambda PaPa: not the mother of all lambda phages. Science 258 1145-8 PubMed
  8. ↑ 8.00 8.01 8.02 8.03 8.04 8.05 8.06 8.07 8.08 8.09 8.10 Edited by Roger W. Hendrix, Jeffrey W. Roberts, Franklin W. Stahl and Robert A. Weisberg (1983) Lambda II (Cold Spring Harbor Laboratory, Cold Spring Harbor, NY)
  9. ↑ 9.00 9.01 9.02 9.03 9.04 9.05 9.06 9.07 9.08 9.09 9.10 Edward K. Wagner et al (2007) Basic Virology (Wiley-Blackwell) ISBN 978-1-4051-4715-6
  10. ↑ Taylor, K & Wegrzyn, G (1995) Replication of coliphage lambda DNA. FEMS Microbiol. Rev. 17 109-19 PubMed
  11. ↑ 11.0 11.1 11.2 Bruce A. Voles (2002) The Biology of Viruses (McGraw-Hill, Boston) ISBN 0-07-237031-9
  12. ↑ Wegrzyn, A et al. (1995) Plasmid and host functions required for lambda plasmid replication carried out by the inherited replication complex. Mol. Gen. Genet. 247 501-8 PubMed
  13. ↑ Neidhardt, FC et al. (1996) Escherichia coli and Salmonella typhimurium: Cellular and Molecular Biology 2nd ed. (ASM Press, Washington, DC)
  14. ↑ Appleyard, RK (1954) Segregation of Lambda Lysogenicity during Bacterial Recombination in Escherichia Coli K12. Genetics 39 429-39 PubMed
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