Hantaviruses are zoonotic viruses transmitted to humans by persistently infected rodents, giving rise to serious outbreaks of hemorrhagic fever with renal syndrome (HFRS) or of hantavirus pulmonary syndrome (HPS), depending on the virus, which are associated with high case fatality rates. There is only limited knowledge about the organization of the viral particles and in particular, about the hantavirus membrane fusion glycoprotein Gc, the function of which is essential for virus entry. We describe here the X-ray structures of Gc from Hantaan virus, the type species hantavirus and responsible for HFRS, both in its neutral pH, monomeric pre-fusion conformation, and in its acidic pH, trimeric post-fusion form. The structures confirm the prediction that Gc is a class II fusion protein, containing the characteristic β-sheet rich domains termed I, II and III as initially identified in the fusion proteins of arboviruses such as alpha- and flaviviruses. The structures also show a number of features of Gc that are distinct from arbovirus class II proteins. In particular, hantavirus Gc inserts residues from three different loops into the target membrane to drive fusion, as confirmed functionally by structure-guided mutagenesis on the HPS-inducing Andes virus, instead of having a single “fusion loop”. We further show that the membrane interacting region of Gc becomes structured only at acidic pH via a set of polar and electrostatic interactions. Furthermore, the structure reveals that hantavirus Gc has an additional N-terminal “tail” that is crucial in stabilizing the post-fusion trimer, accompanying the swapping of domain III in the quaternary arrangement of the trimer as compared to the standard class II fusion proteins. The mechanistic understandings derived from these data are likely to provide a unique handle for devising treatments against these human pathogens.

The consensus sequence for single-base deletions in non-reiterated runs during in vitro DNA-dependent DNA polymerisation is refined using data available in the literature. This leads to the observation that chain termination codons are hotspots for single-base deletions. The evolutionary implications are discussed in two models which differ in whether polymerases evolved while the genetic code emerged or after the genetic code was fixed. A possible answer to the question 'Why are stop codons just what they are?' is suggested.© 1997 Federation of European Biochemical Societies.Key words: Genetic code; Stop codon; Deletion; Polymerase AssumptionsThe mutational spectra of in vitro polymerisation [1-9] for DNA polymerases belonging to families found in at least two of the three living kingdoms [10] are considered here as relevant with respect to a primordial polymerase; we will not take into account DNA polymerases ß, as they are family X DNA polymerases so far only found among eukaryotes [11], and HIV reverse transcriptases, which emerged very 'late' in evolution [12].As it is believed that RNA preceded DNA in evolution [13], data for RNA replicases would be more relevant but are not available; recent evidence shows, however, that DNA and RNA replicases are very closely related [14][15][16]: a single substitution of a hydroxyl group by a hydrogen atom in the Y639F mutant of T7 RNA polymerase allows a DNA replicase to function as a RNA replicase [17], and a single mutation confers on Moloney murine leukaemia virus reverse transcriptase the ability to replicate RNA [18]; we also note that Escherichia coli DNA polymerase I is an accurate RNA-dependent DNA polymerase [19]. Polymerase errorsPolymerase-induced mutations are mainly substitutions and frameshifts [1][2][3][4][5][6][7]. For the Klenow polymerase domain [1], which has no nuclease domain, as can be assumed for a primordial polymerase, the frameshift error rate is about half the substitution error rate: frameshift mutations therefore represent a significant proportion of the mutations in such systems. Frameshifts result mostly from deletions and additions of one base [1][2][3][4][5]. Crucially, these are highly deleterious by preventing translation in the correct reading frame of the codons »Corresponding author. Fax: (44) (1223) 402 140. E-mail: jlj@mrc-lmb.cam.ac.uk downstream of the mutation. Frameshifts occurring in directly repeated and palindromic sequences [8] will be addressed in the discussion. Here, we will focus on frameshifts in non-reiterated runs, where single-base deletions occur far more frequently than single-base additions [1,2,4,7]. Additions will therefore be neglected in the following. For polymerases with and without nuclease domains, the data indicate no significant differences in the consensus sequence for single-base deletions in non-reiterated runs. It has been defined as YR [1], TTR [9], YTG [6] and TR [8]. Using the current data [1-9] we here refine it as YTRV (V = C, A or G; Table 1). These singlebase deletions are found to...