Substitutions of 169Lys and 173Thr in nonstructural protein 1 influence the infectivity and pathogenicity of XJ-160 virus.

An infectious clone (pBR-XJ160) was constructed using the full-length cDNA of the Sindbis-like XJ-160 virus. Two nucleotide mutations, causing amino acid changes at residue 169 from Lys to Arg and at residue 173 from Thr to Ile in the nonstructural protein (nsP) 1 coding region, strongly influenced the infectivity of in vitro-synthesized RNA. We used site-directed mutagenesis to obtain clones encoding a change to Arg at residue 169 of nsP1 (pBR-169), a change to Ile at residue 173 (pBR-173), or both changes (pBR-6973). Infectivity of RNA from pBR-169 was abolished, but viral forms BR-173 and BR-6973 were obtained from pBR-173 and pBR-6973, respectively. Further, BR-173 exhibited higher propagation than BR-XJ160 in cell culture and higher neurovirulence in a suckling mouse model. BR-6973 possessed an intermediate phenotype. BR-173 and BR-6973 showed increased sensitivity to 3-deazaadenosine (3-DZA), which inhibits S-adenosylhomocysteine hydrolase. Thus, mutagenesis at residue 169 in the nsP1 region of XJ-160 is lethal, but mutation at residue 173 from Thr to Ile enhances viral infectivity and neurovirulence and suppresses the lethal effect of the mutation at residue 169. These mutations might be associated with the RNA methyltransferase (MTase) activity of nsP1.