Using cross-linking coupled to matrix-assisted laser desorption/ionization mass spectrometry and CLIP-Seq sequencing, we determined the peptide and oligonucleotide sequences at the interfaces between the capsid proteins and the genomic RNA of bacteriophage MS2. The results suggest that the same coat protein (CP)–RNA and maturation protein (MP)–RNA interfaces are used in every viral particle. The portions of the viral RNA in contact with CP subunits span the genome, consistent with a large number of discrete and similar contacts within each particle. Many of these sites match previous predictions of the locations of multiple, dispersed and degenerate RNA sites with cognate CP affinity termed packaging signals (PSs). Chemical RNA footprinting was used to compare the secondary structures of protein-free genomic fragments and the RNA in the virion. Some PSs are partially present in protein-free RNA but others would need to refold from their dominant solution conformations to form the contacts identified in the virion. The RNA-binding peptides within the MP map to two sections of the N-terminal half of the protein. Comparison of MP sequences from related phages suggests a similar arrangement of RNA-binding sites, although these N-terminal regions have only limited sequence conservation. In contrast, the sequences of the C-termini are highly conserved, consistent with them encompassing pilin-binding domains required for initial contact with host cells. These results provide independent and unambiguous support for the assembly of MS2 virions via a PS-mediated mechanism involving a series of induced-fit viral protein interactions with RNA.

The four brome mosaic virus (BMV) RNAs (RNA1 to RNA4) are encapsidated in three distinct virions that have different disassembly rates in infection. The mechanism for the differential release of BMV RNAs from virions is unknown, since 180 copies of the same coat protein (CP) encapsidate each of the BMV genomic RNAs. Using mass spectrometry, we found that the BMV CP contains a complex pattern of posttranslational modifications. Treatment with phosphatase was found to not significantly affect the stability of the virions containing RNA1 but significantly impacted the stability of the virions that encapsidated BMV RNA2 and RNA3/4. Cryo-electron microscopy reconstruction revealed dramatic structural changes in the capsid and the encapsidated RNA. A phosphomimetic mutation in the flexible N-terminal arm of the CP increased BMV RNA replication and virion produc- T he timing of viral genome release into the infected host cell is critically important to the outcome of infection, as it initiates the race between viral processes and the cellular immune responses against the virus. Even small changes in the timing of viral genome release can result in decreased fitness of the virus (1, 2). The regulation of the release of the viral RNAs is poorly understood, especially for icosahedral viruses.Brome mosaic virus (BMV) is a plant-infecting RNA virus that has served as a model system to study the regulation of RNA virus infection (3). A particularly intriguing feature of BMV is that its tripartite positive-strand RNA genome is encapsidated in three distinct virions. All three virions are required for successful infection. Upon entry into cells, RNA1 and RNA2 direct the translation of the viral replication-associated proteins, while RNA3 encodes the movement protein required for cell-to-cell spread and the coat protein (CP). The CP is translated from subgenomic RNA4. In a typical infection, RNA4 is coencapsidated with RNA3 in a 1:1 ratio (4), although the host species can influence the encapsidation of the viral RNAs (5).The BMV CP has multiple regulatory activities during infection (6, 7). Each BMV capsid contains 180 subunits of the CP arranged in a Tϭ3 symmetry. A CP subunit has structural features similar to those of a histone protein. Both have an intrinsically disordered N-terminal arm rich in positively charged residues followed by sequences that fold into a globular domain (7). The N-terminal arm contributes to the differential encapsidation of BMV RNA, has the ability to translocate from the internal cavity to the outside of the virion, and is preferentially cleaved during proteolysis (8,9). Virions formed by CPs that lack the first 8 residues were found to encapsidate RNA2 well but were labile for the encapsidation of RNA1 (8).The BMV virions can be separated by their density into two populations, one enriched for virions containing RNA1, called B1 virions, and the other enriched for virions that encapsidate RNA2 and RNA3/4, called B2.3/4 virions (9). The B1 virions release RNA1 more rapidly than the B2.3/4 v...

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Brome mosaic virus (BMV) (the Russian strain) infects monocot plants and has been studied extensively in barley and wheat. Here, we report BMV can systemically infect rice (Oryza sativa var. japonica), including cultivars in which the genomes have been determined. The BMV capsid protein can be found throughout the inoculated plants. However, infection in rice exhibits delayed symptom expression or no symptoms when compared with wheat (Triticum aestivum). The sequences of BMV RNAs isolated from rice did not reveal any nucleotide changes in RNA1 or RNA2, while RNA3 had only one synonymous nucleotide change from the inoculum sequence. Preparations of purified BMV virions contained RNA1 at a significantly reduced level relative to the other two RNAs. Analysis of BMV RNA replication in rice revealed that minus-strand RNA1 was replicated at a reduced rate when compared with RNA2. Thus, rice appears to either inhibit RNA1 replication or lacks a sufficient amount of a factor needed to support efficient RNA1 replication.