Figure 1 Thin section electron micrographs of Chikungunya virus in Vero E6 cells. Infected cells showed an abundance of viral particles that tended to associate with the plasma membrane.

(Courtesy of Cynthia Goldsmith, MS and James A. Comer, PhD, CDC.)

Figure 2 Togavirus genomic coding strategies. Shown are comparative schematic representations of the alphavirus and rubivirus genomic RNAs with UTRs represented as solid black lines and ORFs as open boxes (NS-ORF=non-structural protein ORF; S-ORF=structural protein ORF). Within each ORF, the coding sequences for the proteins processed from the translation product of the ORF are delineated. The asterisk between nsP3 and nsP4 in the alphavirus NS-ORF indicates the stop codon present in some alphaviruses that must be translationally read through to produce a precursor containing nsP4. Additionally, within the NS-ORFs, the locations of motifs associated with the following activities are indicated: (Mtr) methyl transferase, (Pro) protease, (Hel) helicase, (X) unknown function, and (Rep) replicase. The sequences encompassed by the sgRNA are also shown.

(Courtesy of T. K. Frey.)

Figure 3 Model for the processing of the alphavirus nonstructural polyprotein during replication. When low levels of P123 are present, cis-cleavage of P1234 generates the minus-strand RNA replicase of the virus. This results in primarily negative sense RNA being generated from the incoming genomic RNA of the virus (upper panel). As the level of the trans-acting protease P123 rises in the infected cell, cleavage in trans generates other RNA replicase complexes. This results in a shift by the virus from the production of primarily negative sense RNA to primarily positive sense RNA. Eventually, replicase complexes capable of producing negative sense RNA will no longer be present in the infected cell resulting in the complete cessation of negative sense RNA synthesis (lower panel). The presence of a leaky opal termination codon (depicted as a black diamond) in the virus genome is believed to lead to a more rapid buildup of P123 in the infected cell, and thus a more rapid conversion to the production of positive sense RNA by the virus.

(Courtesy of Dr Kevin Myles; modified from Powers, A.M. (2008). Togaviruses: Alphaviruses. In: Encyclopedia of Virology, 3rd edn (B.W.J. Mahy and M.H.V. Van Regenmortel, Eds.), Oxford, Elsevier, pp. 96-100.)

Figure 4 Unrooted phylogenetic tree of representative isolates of all alphavirus species generated from the E1 nucleotide sequences using the F84 algorithm of the neighbor-joining program (SESV and RUBV are not included because no homologous sequence for this region is available). Virus abbreviations are from the list of species in the genus Alphavirus. Antigenic complexes are indicated by colored circles.

(From Powers, A.M. (2008). Togaviruses: Alphaviruses. In: Encyclopedia of Virology, 3rd edn (B.W.J. Mahy and M.H.V. Van Regenmortel, Eds.), Oxford, Elsevier, pp. 96-100.)