|Bacteriophage MS2 capsid|
|Group:||Group IV ((+)ssRNA)|
In 1976, the MS2 genome was the first genome to be completely sequenced. This was accomplished by Walter Fiers 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.
Genome and gene products
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, the coat protein, and the replicase protein. However, the expression of these proteins is regulated by complex interplay between translation and RNA secondary structure.
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 (pI) of 3.9.
The structure of the coat protein is a five-stranded β-sheet with two α-helices and a hairpin. When the capsid is assembled, the helices and hairpin face the exterior of the particle, while the β-sheet faces the interior.
MS2 infects only "male" E. coli bacteria, that is, those bearing an F pilus. 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 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 get transiently opened as ribosomes 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", 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, 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 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 and breakdown of the cell wall.
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