Characterization of leader RNA sequences on the virion and mRNAs of mouse hepatitis virus, a cytoplasmic RNA virus.

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Abstract
Mouse hepatitis virus, which replicates in cytoplasm, contains leader RNA sequences at the 5'' end of the virus-specific mRNA. This leader RNA was sequenced by synthesizing c[complementary]DNA from a synthetic oligodeoxyribonucleotide primer (15-mer) that is complementary to the sequences at the junction site between the leader and body sequences of the mRNA. The leader sequences on each mRNA have exactly the same size, which span .apprx. 70 nucleotides. Leader cDNA fragments obtained from several mRNA species were sequenced and found to be identical. Computer analysis of the leader RNA sequences shows that they share extensive sequence homology with the long-terminal repeat region of several mammalian sarcoma viruses, suggesting possible common functions. This is a novel case of spliced leader sequences in the mRNA of a cytoplasmic virus. An identical leader sequence is also present at the 5'' end of the virion genomic RNA. The leader RNA is thus probably encoded by the virion genomic RNA template and is fused to the different body sequences of the various mRNA. Since conventional RNA splicing is not involved, a novel mechanism for fusing 2 noncontiguous RNA segments in the cytoplasm must be utilized during viral transcription. Several minor cDNA bands longer than the leader were also synthesized, suggesting the possible presence of partially homologous sequences in other parts of the genome RNA.