Escherichia virus MS2

Schematic drawing of a Levivirus virion (cross section and side view)

An MS2 virion (viral particle) is about 27 nm in diameter, as determined by electron microscopy.[8] 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.[9] The virion has an isoelectric point (pI) of 3.9.[10]

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.[11]

Escherichia virus MS2 genome

The MS2 genome is one of the smallest known, consisting of 3569 nucleotides of single-stranded RNA.[4] It encodes just four proteins: the maturation protein (A-protein), the lysis protein, the coat protein, and the replicase protein.[1] The gene encoding lysis protein (lys) overlaps both the 3'-end of the upstream gene (cp) and the 5'-end of the downstream gene (rep), and was one of the first known examples of overlapping genes. The positive-stranded RNA genome serves as messenger RNA, and is translated upon viral uncoating within the host cell. Although the four proteins are encoded by the same messenger/viral RNA, they are not all expressed at the same levels; expression of these proteins is regulated by a complex interplay between translation and RNA secondary structure.

MS2 infects enteric bacteria carrying the fertility (F) factor, a plasmid that allows cells to serve as DNA donors in bacterial conjugation. Genes on the F plasmid lead to the production of an F pilus, which serves as the viral receptor. MS2 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 viral RNA has entered the cell, 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 be 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.[1]

Bacteriophage MS2 life cycle

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.[1]

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.[1]

Bacterial lysis and release of newly formed virions occurs when sufficient lysis protein has accumulated. Lysis (L) protein forms pores in the cytoplasmic membrane, which leads to loss of membrane potential and breakdown of the cell wall.[1] The lysis protein is known to bind to DnaJ via an important P330 residue.[12] A LS dipeptide motif on the L protein is found throuout the genus Levivirus and appears to be essential to the lysis activity, although their different locations suggest that they have evolved independently.[13]