Mutations Affecting Transcriptional Termination in the P Gene End of Subacute Sclerosing Panencephalitis Viruses

Abstract

ABSTRACT Numerous mutations are found in subacute sclerosing panencephalitis (SSPE) viruses, and the M gene is the gene most commonly affected. In some SSPE viruses, such as the MF, Osaka-1, Osaka-2, and Yamagata-1 strains, translation of the M protein is complicated by a transcriptional defect that leads to an almost exclusive synthesis of dicistronic P-M mRNA. To understand the molecular mechanisms of this defect, we sequenced the P gene at the P-M gene junction for several virus strains and probed the involvement of several mutations in the readthrough region via their expression in measles virus minigenomes containing different sequences of the P-M gene junction and flanking reporter genes. The deletion of a single U residue in the U tract of the Osaka-1 strain (3′-UAAUAUUUUU-5′) compared with the consensus sequence resulted in a marked reduction of the expression of the downstream reporter gene. In addition, the expression of the downstream gene was markedly decreased by (i) the substitution of a C residue in the U tract of the P gene end of the OSA-2/Fr/B strain of the Osaka-2 virus (3′-UGAUAUUCUU-5′ compared with the sequence 3′-UGAUAUUUUU-5′ from a sibling virus of the same strain, OSA-2/Fr/V), and (ii) the substitution of a G in the sequence of the P gene end of the Yamagata-1 strain at a variable site immediately upstream from the six-U tract (3′-UGAUGUUUUUU-5′ instead of 3′-UGAUUUUUUUU-5′). Mutations at the P gene end can account for the readthrough transcription variation at the P-M gene junction, which directly affects M protein expression.