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|Blue: Coat Protein pIII; Brown: Coat Proteín pVI; Red: Coat Protein pVII; Limegreen: Coat Protein pVIII; Fuchsia: Coat Proteín pIX; Purple: Single Stranded DNA|
|Group:||Group II (ssDNA)|
M13 is a filamentous bacteriophage composed of circular single stranded DNA (ssDNA) which is 6407 nucleotides long encapsulated in approximately 2700 copies of the major coat protein P8, and capped with 5 copies of two different minor coat proteins (P9, P6, P3) on the ends. The minor coat protein P3 attaches to the receptor at the tip of the F pilus of the host Escherichia coli. Infection with filamentous phages is not lethal; however, the infection causes turbid plaques in E. coli. It is a non-lytic virus. However a decrease in the rate of cell growth is seen in the infected cells. M13 plasmids are used for many recombinant DNA processes, and the virus has also been studied for its uses in nanostructures and nanotechnology.
The phage coat is primarily assembled from a 50 amino acid protein called pVIII (or p8), which is encoded by gene VIII (or g8) in the phage genome. For a wild type M13 particle, it takes approximately 2700 copies of p8 to make the coat about 900 nm long. The coat's dimensions are flexible though and the number of p8 copies adjusts to accommodate the size of the single stranded genome it packages. For example, when the phage genome was mutated to reduce its number of DNA bases (from 6.4 kb to 221 bp), then the number of p8 copies was decreased to fewer than 100, causing the p8 coat to shrink in order to fit the reduced genome. The phage appear to be limited at approximately twice the natural DNA content. However, deletion of a phage protein (p3) prevents full escape from the host E. coli, and phage that are 10-20X the normal length with several copies of the phage genome can be seen shedding from the E. coli host.
There are four other proteins on the phage surface, two of which have been extensively studied. At one end of the filament are five copies of the surface exposed pIX (p9) and a more buried companion protein, pVII (p7). If p8 forms the shaft of the phage, p9 and p7 form the "blunt" end that is seen in the micrographs. These proteins are very small, containing only 33 and 32 amino acids respectively, though some additional residues can be added to the N-terminal portion of each which are then presented on the outside of the coat. At the other end of the phage particle are five copies of the surface exposed pIII (p3) and its less exposed accessory protein, pVI (p6). These form the rounded tip of the phage and are the first proteins to interact with the E. coli host during infection. p3 is also the last point of contact with the host as new phage bud from the bacterial surface.
The general stages to a viral life cycle are: infection, replication of the viral genome, assembly of new viral particles and then release of the progeny particles from the host. Filamentous phage use a bacterial structure known as the F pilus to infect E. coli, with the M13 p3 tip contacting the TolA protein on the bacterial pilus. The phage genome is then transferred to the cytoplasm of the bacterial cell where resident proteins convert the single stranded DNA genome to a double stranded replicative form ("RF"). This DNA then serves as a template for expression of the phage genes.
Two phage gene products play critical roles in the next stage of the phage life cycle, namely amplification of the genome. pII (aka p2) nicks the double stranded form of the genome to initiate replication of the + strand. Without p2, no replication of the phage genome can occur. Host enzymes copy the replicated + strand, resulting in more copies of double stranded phage DNA. pV (aka p5) competes with double stranded DNA formation by sequestering copies of the + stranded DNA into a protein/DNA complex destined for packaging into new phage particles. Interestingly there is one additional phage-encoded protein, pX (p10), that is important for regulating the number of double stranded genomes in the bacterial host. Without p10 no + strands can accumulate. What's particularly interesting about p10 is that it's identical to the C-terminal portion of p2 since the gene for p10 is within the gene for p2 and the protein arises from transcription initiation within gene 2. This makes the manipulation of p10 inextricably linked to manipulation of p2 (an engineering headache) but it also makes for a compact and efficient phage in nature.
Phage maturation requires the phage-encoded proteins pIV (p4), pI (p1) and its translational restart product pXI (p11). Multiple copies (on the order of 12 or 14) of p4 assemble in the outer membrane into a stable, i.e. detergent resistant, barrel-shaped structure. Similarly a handful of the p1 and p11 proteins (5 or 6 copies of each) assemble in the bacterial inner membrane, and genetic evidence suggests C-terminal portions of p1 and p11 interact with the N-terminal portion of p4 in the periplasm. Together the p1, p11, p4 complex forms channels through which mature phage are secreted from the bacterial host.
To initiate phage secretion, two of the minor phage coat proteins, p9 and p7, are thought to interact with the p5-single stranded DNA complex at a region of the DNA called the packaging sequence (aka PS). The p5 proteins covering the single stranded DNA are then replaced by p8 proteins that are embedded in the bacterial membrane and the growing phage filament is threaded through the p1, p11, p4 channel. This replacement of p5 by p8 explains the microphage data presented earlier indicate how the size of the phage particle is determined by the number of bases the phage packages. Once the phage DNA has been fully coated with p8, the secretion terminates by adding the p3/p6 cap, and the new phage detaches from the bacterial surface.
Replication in E. coli
Below are steps involved with replication of M13 in E. coli.
- Viral (+) strand DNA enters cytoplasm
- Complementary (-) strand is synthesized by bacterial enzymes
- DNA Gyrase, a type II topoisomerase, acts on double-stranded DNA and catalyzes formation of negative supercoils in double-stranded DNA
- Final product is parental replicative form (RF) DNA
- A phage protein, pII, nicks the (+) strand in the RF
- 3'-hydroxyl acts as a primer in the creation of new viral strand
- pII circulizes displaced viral (+) strand DNA
- Pool of progeny double-stranded RF molecules produced
- Negative strand of RF is template of transcription
- mRNAs are translated into the phage proteins
Phage proteins in the cytoplasm are pII, pX, and pV, and they are part of the replication process of DNA. The other phage proteins are synthesized and inserted into the cytoplasmic or outer membranes.
- pV dimers bind newly synthesized single-stranded DNA and prevent conversion to RF DNA
- RF DNA synthesis continues and amount of pV reaches critical concentration
- DNA replication switches to synthesis of single-stranded (+) viral DNA
- pV-DNA structures from about 800 nm long and 8 nm in diameter
- pV-DNA complex is substrate in phage assembly reaction
George Smith, among others, showed that fragments of EcoRI endonuclease could be fused in the unique Bam site of f1 filamentous phage and thereby expressed in gene III whose protein pIII was externally accessible. M13 does not have this unique Bam site in gene III. M13 had to be engineered to have accessible insertion sites, making it limited in its flexibility in handling different sized inserts. Because the M13 phage display system allows great flexibility in the location and number of recombinant proteins on the phage, it is a popular tool to construct or serve as a scaffold for nanostructures. For example, the phage can be engineered to have a different protein on each end and along its length. This can be used to assemble structures like gold or cobalt oxide nano-wires for batteries or to pack carbon nanotubes into straight bundles for use in photovoltaics.
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- 20.109(S07): Start-up Genome Engineering 
- Phage Display: A Laboratory Manual
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