Bacteriophage MS2

Bacteriophage MS2 (Emesvirus zinderi), commonly called MS2, is an icosahedral, positive-sense single-stranded RNA virus that infects the bacterium Escherichia coli and other members of the Enterobacteriaceae. MS2 is a member of a family of closely related bacterial viruses that includes bacteriophage f2, bacteriophage Qβ, R17, and GA.

It is small and contains a maturation protein, coat protein, and genomic RNA. It also has one of the smallest known genomes, encoding four proteins.

The MS2 lifecycle involves infecting bacteria with the fertility factor, enabling the virus to attach to the pilus, though the mechanism by which the virus's RNA enters the bacterium remains unknown. Once inside, the viral RNA starts functioning as a messenger RNA to produce viral proteins. MS2 replicates its plus-strand genome by creating a minus strand RNA as a template. The virus then assembles, and the bacterial cell lyses, releasing new viruses.

The virus was isolated in 1961 and its genome was the first to be fully sequenced, in 1976, providing a crucial understanding of genetic codes. In practical applications, MS2's structural components have been used to detect RNA in living cells. The virus is also under research for potential uses in drug delivery, tumor imaging, and light harvesting. Furthermore, because of its structural similarities to noroviruses, its preferred proliferation conditions, and its lack of pathogenicity to humans, MS2 serves as a substitute in studies of norovirus disease transmission.

The MS2 genome is one of the smallest known, consisting of 3569 nucleotides of single-stranded RNA. It encodes just four proteins: the maturation protein (A-protein), the lysis (lys) protein, the coat protein (cp), and the replicase (rep) protein. The gene encoding 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 a 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.

An MS2 virion (viral particle) is about 27 nm in diameter, as determined 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 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 specifies the proteins of the F pilus, which includes the F-pilin protein that serves as the viral receptor. MS2 attaches to the F-pilin on the side of the pilus using its single maturation protein.