Bunyaviricetes
Bunyaviricetes is a class of enveloped viruses possessing linear, negative-sense, single-stranded RNA genomes that are segmented, ranging from tripartite to polysegmented configurations, and classified within the phylum Negarnaviricota of the kingdom Orthornavirae.[1] Established by the International Committee on Taxonomy of Viruses (ICTV) in April 2024 through the elevation of the former order Bunyavirales to class rank, this taxonomic group accommodates the rapidly expanding diversity of related viruses discovered primarily through metagenomics.[1] Virions are generally spherical or pleomorphic, measuring 80–120 nm in diameter, and feature a lipid envelope studded with glycoprotein projections approximately 5–10 nm long.[2] The class Bunyaviricetes currently includes two orders: Elliovirales, comprising seven families such as Peribunyaviridae, Phasmaviridae, and Tospoviridae; and Hareavirales, which encompasses eight families including Arenaviridae, Nairoviridae, and Phenuiviridae.[1] These viruses exhibit broad host tropism, infecting vertebrates (including humans and livestock), invertebrates (such as arthropods), plants, fungi, and protists, with transmission often occurring via vectors like mosquitoes, ticks, sandflies, or rodents.[3] Genome segments typically encode essential proteins including the RNA-dependent RNA polymerase on the large (L) segment, glycoproteins on the medium (M) segment, and the nucleocapsid protein on the small (S) segment, though some families display ambisense coding strategies or additional non-structural proteins that modulate host immune responses and replication.[3] Members of Bunyaviricetes are of significant medical, veterinary, and agricultural importance, with many acting as emerging pathogens responsible for severe diseases.[4] Notable human pathogens include Crimean-Congo hemorrhagic fever virus (Nairoviridae), Rift Valley fever virus (Phenuiviridae), and hantaviruses (Hantaviridae), which cause viral hemorrhagic fevers characterized by fever, bleeding, and multi-organ failure, often with high case-fatality rates and spread via arthropod bites or rodent exposure.[4] In agriculture, tospoviruses (Tospoviridae) and other plant-infecting members lead to substantial crop losses in tomatoes, peppers, and peanuts, transmitted by thrips.[3] The ecological complexity of these viruses, including their segmented genomes that facilitate reassortment and evolution, underscores their potential for zoonotic spillover and pandemic threats.[1]General Characteristics
Virion Structure
Bunyaviricetes virions are typically enveloped particles that exhibit spherical or pleomorphic morphology, with diameters ranging from 80 to 120 nm. The envelope consists of a lipid bilayer acquired from host cell membranes during budding, which surrounds the internal components. This structure is conserved across most families within the class, though some members, such as those in Phasmaviridae, may form tubular virions measuring up to 600 nm in length.[5][6] Embedded within the viral envelope are surface glycoproteins, primarily Gn and Gc, which form heterodimers or spikes protruding 5-18 nm from the lipid bilayer. These glycoproteins play essential roles in host cell receptor binding and low-pH-induced membrane fusion during entry, with Gn often mediating attachment to receptors like DC-SIGN or LRP1, while Gc facilitates the fusion process in endosomes. In certain families like Phenuiviridae, Gn additionally contributes to immune evasion by interacting with host factors such as STING to inhibit NF-κB signaling and suppress type I interferon responses.[5][7][8] Internally, the virions contain helical nucleocapsids composed of the nucleoprotein (N) that encapsidates the segmented RNA genome, forming ribonucleoprotein complexes. These nucleocapsids display a characteristic helical symmetry, with the N protein adopting a two-lobed structure that clamps the RNA, promoting ordered assembly; dimensions and exact symmetry vary by order, such as more elongated, rod-like forms in Hareavirales, where ribonucleoproteins can extend as parallel filaments up to 10 nm thick.[9][10][11] Unlike many other negative-sense RNA viruses, most Bunyaviricetes lack a dedicated matrix protein to bridge the envelope and nucleocapsids, relying instead on interactions between glycoprotein cytoplasmic tails and the ribonucleoproteins for virion stability and assembly; exceptions occur in Arenaviridae, where the Z protein serves a matrix-like function.[12][13]Genome Organization
The genomes of viruses classified within the Bunyaviricetes consist of a tripartite, single-stranded RNA structure comprising three segments—large (L), medium (M), and small (S)—with a total length of 11–19 kb. These segments are primarily of negative-sense polarity, though ambisense coding strategies occur in the M and/or S segments of certain genera, such as Phlebovirus (in the family Phenuiviridae). The L segment encodes the RNA-dependent RNA polymerase (RdRp), the core enzyme for viral genome replication and transcription. The M segment encodes glycoproteins (Gn and Gc) derived from precursor polyproteins, along with non-structural proteins like NSm. The S segment encodes the nucleoprotein (N), which associates with the genomic RNA to form the ribonucleoprotein complex, and in some cases, a non-structural protein (NSs).[2] Segment lengths vary across families but follow characteristic ranges: the L segment is typically 6.5–8.5 kb, the M segment 3.5–6.5 kb, and the S segment 1–3.5 kb. In ambisense configurations, prevalent in families like Phenuiviridae, the M and S segments utilize internal transcription initiation sites to produce two non-overlapping open reading frames in opposite polarities, enabling the synthesis of both genomic-sense and antigenomic-sense proteins from a single template without requiring splicing. This strategy contrasts with the strictly negative-sense coding in the L segment and in non-ambisense S segments of other families, such as Orthobunyavirus. For instance, in Rift Valley fever virus (Phlebovirus genus, Phenuiviridae family), the S segment's ambisense arrangement allows simultaneous expression of N from the negative-sense strand and NSs from the positive-sense strand.[2][3] Each segment possesses conserved, complementary nucleotide sequences at the 5' and 3' termini, which base-pair to form panhandle structures that serve as promoters for replication initiation by the RdRp. These terminal motifs are genus-specific but often share partial conservation; for example, many orthobunyaviruses feature a 5'-AGCAGUU-3' sequence at the 5' end, complementary to the 3' end (3'-UCGUCA A-5'), enabling circularization and efficient RNA synthesis. Similar panhandle formations occur across the class, with variations like 5'-ACAGAUU-3' in phleboviruses, underscoring their role in stabilizing the genome and directing polymerase binding.[2][14] Bunyaviricetes exhibit substantial genetic diversity, particularly among arthropod-borne members, driven by high nucleotide substitution rates of $10^{-4} to $10^{-3} substitutions per site per year. These elevated rates, observed in viruses like severe fever with thrombocytopenia syndrome virus, reflect adaptations to alternating transmission between invertebrate vectors and vertebrate hosts, contrasting with lower rates in non-arthropod-borne lineages such as hantaviruses.[15][16]Replication Cycle
Viral Entry and Replication
Bunyaviricetes viruses initiate infection through receptor-mediated attachment to host cells, primarily via surface glycoproteins such as Gn and Gc. For instance, some orthobunyaviruses and nairoviruses utilize DC-SIGN or L-SIGN as entry receptors, while hantaviruses bind β3 integrins or decay-accelerating factor (DAF/CD55). Following attachment, the virions enter host cells via clathrin-dependent or clathrin-independent endocytosis, leading to trafficking within endosomes where low pH (approximately 4.9–6.3) triggers conformational changes in the glycoproteins, resulting in fusion of the viral envelope with the endosomal membrane and release of the ribonucleoprotein complex into the cytoplasm.[17] Upon entry, replication occurs entirely in the cytoplasm, often within Golgi-derived vesicles that provide a protected environment for viral RNA synthesis. The virion-associated RNA-dependent RNA polymerase (RdRp), along with nucleoprotein (N) and other cofactors, initiates primary transcription using the negative-sense genomic RNA segments as templates to produce viral mRNAs. These mRNAs acquire a 5' cap through a cap-snatching mechanism, where the viral endonuclease domain of the L protein cleaves 10–20 nucleotide leader sequences from host mRNAs, enabling efficient translation by host ribosomes.[18][19] After primary transcription, the replication cycle switches to full-length antigenome synthesis, where positive-sense complementary RNAs (cRNAs) are produced from the genomic RNAs and serve as templates for new negative-sense genomic RNA synthesis. In families with ambisense genome segments, such as Phenuiviridae, viral mRNAs are produced by transcription from both the genomic and antigenomic RNA templates, allowing expression of two non-overlapping open reading frames separated by an intergenic region.[20] Many Bunyaviricetes employ a non-structural protein NSs to counteract host antiviral defenses by inducing shutoff of host gene expression, particularly inhibiting the interferon response. NSs achieves this through degradation or sequestration of host transcription factors, such as components of TFIIH or RNA polymerase II, thereby suppressing interferon-β production and promoting viral replication. For example, in Rift Valley fever virus (Phenuiviridae), NSs promotes the degradation of TFIIH subunit p62 to disrupt general host transcription and suppress interferon production.[21]Assembly and Exit
In bunyaviricetes, nucleocapsid assembly occurs in the cytoplasm, where the nucleoprotein (N) encapsidates each genomic RNA segment independently to form ribonucleoprotein complexes (RNPs) within specialized virus factories often located near the Golgi apparatus.[22] These RNPs associate with the viral RNA-dependent RNA polymerase (RdRp) for initial packaging signals, facilitating their subsequent incorporation into nascent virions.[22] The process involves interactions between the RNPs and the cytoplasmic tails of glycoproteins, with evidence from Rift Valley fever virus (RVFV) indicating non-segment-specific packaging where the M segment may play a central role.[22] Viral glycoproteins Gn and Gc, synthesized as precursors in the endoplasmic reticulum (ER), undergo processing and heterodimerization before trafficking to the Golgi apparatus via the secretory pathway.[22] In the Golgi, these glycoproteins form spikes on the viral envelope and drive the budding of RNPs into Golgi-derived vesicles, allowing the virions to acquire a host-derived lipid envelope embedded with Gn-Gc heterodimers.[22] This envelopment process, observed in viruses like severe fever with thrombocytopenia syndrome virus (SFTSV), may also involve the ER-Golgi intermediate compartment (ERGIC) or autophagosome-like structures in some cases, with endosomal sorting complexes required for transport (ESCRT) contributing to membrane scission.[22] Mature virions are released from infected cells through exocytosis of the Golgi-derived vesicles, completing the egress phase of the replication cycle.[22] Post-budding maturation includes glycoprotein processing, such as cleavage by furin-like or subtilisin/kexin isozyme-1/site-1 protease (SKI-1/SPase) in families like Nairoviridae (e.g., Crimean-Congo hemorrhagic fever virus), which is essential for infectivity.[22] In orthobunyaviruses like Bunyamwera virus (BUNV), maturation involves glycan trimming in the Golgi trans-compartments.[22] During co-infection with related strains, bunyaviricetes exhibit segment reassortment, where RNPs from different parental viruses are packaged into progeny virions, promoting genetic diversity.[22] Experimental models of orthobunyavirus co-infections, such as Batai and Bunyamwera viruses in mammalian cells, demonstrate reassortment frequencies up to approximately 7-10%, with diploid (mixed-segment) viruses also detected at rates around 25%.[23] This mechanism contributes to rapid evolution and emergence of novel variants in natural settings.[22]Taxonomy and Evolution
Classification Hierarchy
Bunyaviricetes is a class of viruses within the phylum Negarnaviricota and the realm Riboviria, encompassing negative-sense single-stranded RNA viruses characterized primarily by segmented genomes, typically tripartite.[1] In April 2024, the International Committee on Taxonomy of Viruses (ICTV) promoted the former order Bunyavirales to the rank of class Bunyaviricetes to better accommodate the rapid expansion of related polyploviricotine viruses, reflecting their phylogenetic diversity and increasing discoveries across diverse hosts.[1] The class currently comprises two orders: Elliovirales and Hareavirales. As of 2025, Bunyaviricetes includes 15 families distributed across these orders, with ongoing additions driven by metagenomic surveys. For instance, the order Elliovirales features prominent families such as Peribunyaviridae and Hantaviridae, while Hareavirales encompasses Nairoviridae and Phenuiviridae.[1][24]| Order | Example Families | Key Characteristics and Examples |
|---|---|---|
| Elliovirales | Peribunyaviridae, Hantaviridae, Tospoviridae, Fimoviridae, Cruliviridae, Phasmaviridae | Arthropod- and rodent-borne viruses with tri-segmented genomes; plant-infecting viruses transmitted by insects; e.g., genus Orthobunyavirus (La Crosse virus), genus Hantavirus (Sin Nombre virus), genera in Tospoviridae affecting crops.[25][1] |
| Hareavirales | Phenuiviridae, Nairoviridae, Mypoviridae | Diverse hosts including vertebrates and invertebrates; e.g., genus Phlebovirus (Rift Valley fever virus), recent additions like Mypoviridae with three-segment genomes.[1][26] |