Bacteriophage MS2
The bacteriophage MS2 is an icosahedral, positive-sense single-stranded RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 virus that infects the bacterium Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...



In 1961, MS2 was isolated by Alvin John Clark and recognized as an RNA-containing phage very similar to bacteriophage f2
Bacteriophage f2
The bacteriophage f2 is an icosahedral, positive-sense single-stranded RNA virus that infects the bacterium Escherichia coli. It is extremely similar to bacteriophage MS2.-History:...

In 1976, MS2 was the first organism to have its genome completely sequenced. This was accomplished by Walter Fiers
Walter Fiers
Walter Fiers is a Belgian molecular biologist.He obtained a degree of Engineer for Chemistry and Agricultural Industries at the University of Ghent in 1954, and started his research career as an enzymologist in the laboratory of Laurent Vandendriessche in Ghent. In 1956-57, he worked with Heinz...

 and his team, building upon their earlier milestone in 1972 of the first gene to be completely sequenced, the MS2 coat protein. These sequences were determined at the RNA level, whereas the next landmark achievement, the sequence of the bacteriophage ΦX174 genome in 1977, was determined using DNA.


The MS2 genome is one of the smallest known, 3569 nucleotides long. It encodes just four genes: the maturation protein (A-protein), the lysis protein, and the replicase protein. However, the expression of these proteins is regulated by complex interplay between translation and RNA secondary structure
Nucleic acid secondary structure
The secondary structure of a nucleic acid molecule refers to the basepairing interactions within a single molecule or set of interacting molecules, and can be represented as a list of bases which are paired in a nucleic acid molecule....


An MS2 virion is about 27 nm in diameter (by electron microscopy). It consists of one copy of the maturation protein and 180 copies of the coat protein (organized as 90 dimers) arranged into an icosahedral shell with triangulation number T=3, protecting the genomic RNA inside. The virion has an isoelectric point
Isoelectric point
The isoelectric point , sometimes abbreviated to IEP, is the pH at which a particular molecule or surface carries no net electrical charge....

 (pI) of 3.9.

MS2 infects only "male" E. coli bacteria, that is, those bearing an F pilus
right|thumb|350px|Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell...

. It attaches to the side of the pilus via its single maturation protein. The precise mechanism by which phage RNA enters the bacterium is unknown. Once the RNA is inside, it begins to function as a messenger RNA
Messenger RNA
Messenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...

 for the production of phage proteins. The gene for the most abundant protein, the coat protein, can be immediately translated. The translation start of the replicase gene is normally hidden within RNA secondary structure, but can transiently opened as ribosomes
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....

 pass through the coat protein gene. Replicase translation is also shut down once large amounts of coat protein have been made; coat protein dimers bind and stabilize the RNA "operator hairpin
Stem-loop intramolecular base pairing is a pattern that can occur in single-stranded DNA or, more commonly, in RNA. The structure is also known as a hairpin or hairpin loop. It occurs when two regions of the same strand, usually complementary in nucleotide sequence when read in opposite directions,...

", blocking the replicase start. The start of the maturation protein gene is accessible in RNA being replicated but hidden within RNA secondary structure in the completed MS2 RNA; this ensures translation of only a very few copies of maturation protein per RNA. Finally, the lysis protein gene can only be initiated by ribosomes that have completed translation of the coat protein gene and "slip back" to the start of the lysis protein gene, at about a 5% frequency.

Replication of the plus-strand MS2 genome requires synthesis of the complementary minus strand RNA, which can then be used as a template for synthesis of a new plus strand RNA. MS2 replication has been much less well studied than replication of the highly related bacteriophage Qβ
Bacteriophage Qβ
Bacteriophage Qβ is an icosahedral virus with a diameter of 25 nm. Its host is Escherichia coli. Qβ enters its host cell through the side of the F pilus.-Genetics:...

, partly because the MS2 replicase has been difficult to isolate, but is likely to be similar.

The formation of the virion is thought to be initiated by binding of maturation protein to the MS2 RNA; in fact, the complex of maturation protein and RNA is infectious. The assembly of the icosahedral shell or capsid
A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...

 from coat proteins can occur in the absence of RNA; however, capsid assembly is nucleated by coat protein dimer binding to the operator hairpin, and assembly occurs at much lower concentrations of coat protein when MS2 RNA is present.

Bacterial lysis and release of newly formed virions occurs when sufficient lysis protein has accumulated. Lysis protein forms pores in the cytoplasmic membrane, which leads to loss of membrane potential
Membrane potential
Membrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...

 and breakdown of the cell wall.

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