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A hallmark of negative-strand RNA viruses (NSVs) is that their genomes never exist as free RNA, but instead are always assembled with many copies of a single nucleoprotein (N) to form highly stable nucleocapsids. Moreover, viral genomes are the only RNAs in infected cells that are assembled with N. The mechanism by which this specific association occurs, for both the segmented (s) and non-segmented (ns) viruses, has recently become clearer due to our expanding knowledge of N protein and nucleocapsid structures., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Orthomyxovirus Influenza A virus (IAV) heterotrimeric polymerase performs transcription of viral mRNAs by cap-snatching, which involves generation of capped primers by host pre-mRNA binding via the PB2 subunit cap-binding site and cleavage 10–13 nucleotides from the 5′ cap by the PA subunit endonuclease. Thogotoviruses, tick-borne orthomyxoviruses that includes Thogoto (THOV), Dhori (DHOV) and Jos (JOSV) viruses, are thought to perform cap-snatching by cleaving directly after the cap and thus have no heterogeneous, host-derived sequences at the 5′ extremity of their mRNAs. Based on recent work identifying the cap-binding and endonuclease domains in IAV polymerase, we determined the crystal structures of two THOV PB2 domains, the putative cap-binding and the so-called ‘627-domain’, and the structures of the putative endonuclease domains (PA-Nter) of THOV and DHOV. Despite low sequence similarity, corresponding domains have the same fold confirming the overall architectural similarity of orthomyxovirus polymerases. However the putative Thogotovirus cap-snatching domains in PA and PB2 have non-conservative substitutions of key active site residues. Biochemical analysis confirms that, unlike the IAV domains, the THOV and DHOV PA-Nter domains do not bind divalent cations and have no endonuclease activity and the THOV central PB2 domain does not bind cap analogues. On the other hand, sequence analysis suggests that other, non-influenza, orthomyxoviruses, such as salmon anemia virus (isavirus) and Quaranfil virus likely conserve active cap-snatching domains correlating with the reported occurrence of heterogeneous, host-derived sequences at the 5′ end of the mRNAs of these viruses. These results highlight the unusual nature of transcription initiation by Thogotoviruses. [ABSTRACT FROM AUTHOR]

Commentary on Tawar, R.G.; Duquerroy, S.; Vonrhein, C.; Varela, P.F.; Damier-Piolle, L.; Castagné, N.; MacLellan, K.; Bedouelle, H.; Bricogne, G.; Bhella, D.; Eléouët, J.-F.; Rey, F.A. Crystal structure of a nucleocapsid-like nucleoprotein-RNA complex of respiratory syncytial virus. [ABSTRACT FROM AUTHOR]

Vesicular stomatitis virus (VSV), the founding member of the mononegavirus order (Mononegavirales), was found to be a negative strand RNA virus in the 1960s, and since then the number of such viruses has continually increased with no end in sight. Sendai virus (SeV) was noted soon afterwards due to an outbreak of newborn pneumonitis in Japan whose putative agent was passed in mice, and nowadays this mouse virus is mainly the bane of animal houses and immunologists. However, SeV was important in the study of this class of viruses because, like flu, it grows to high titers in embryonated chicken eggs, facilitating the biochemical characterization of its infection and that of its nucleocapsid, which is very close to that of measles virus (MeV). This review and opinion piece follow SeV as more is known about how various mononegaviruses express their genetic information and carry out their RNA synthesis, and proposes a unified model based on what all MNV have in common. [ABSTRACT FROM AUTHOR]