The coronavirus leader-primed transcription model proposes that free leader RNA species derived from the 5′-end of the genomic RNA are utilized as a primer for the transcription of subgenomic mRNAs. To elucidate the precise mechanism of leader-priming, we cloned and sequenced the 5′-end of the mouse hepatitis virus genomic RNA. The 5′-terminal sequences are identical to the leader sequences present at the 5′-end of the subgenomic mRNAs. Two possible hairpin loop structures and an AU-rich region around the 3′-end of the leader sequence may provide the termination site for leader RNA synthesis. The comparison of 5′-end genomic sequences and the intergenic start sites for mRNA transcription revealed that there are homologous regions of 7-18 nucleotides at the putative leader/body junction sites. Some intergenic regions contain a mismatching nucleotide within this homologous region. We propose that free leader RNA binds to the intergenic region due to this homology and is cleaved at the mismatching nucleotide before serving as a primer. Thus, the free leader RNA species may be longer than the leader sequences in the subgenomic mRNAs and different mRNAs may have different leader/body junction sites.
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