Identification of a Noncanonical Transcription Initiation Site for Transcription of a Subgenomic mRNA of Mouse Hepatitis Virus
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Subgenomic mRNA transcription of mouse hepatitis virus (MHV) involves the interaction between the leader and the intergenic (IG) sequence of the template. All mRNAs contain multiple open reading frames (ORF) with the exception of the smallest mRNA. In general, only the 5’-most ORF of each mRNA is translated into a protein via the cap-dependent ribosomal scanning mechanism, while the down-stream ORFs are not translatable. The E (envelope) protein of MHV is a structural protein (Yu et al., 1994), essential for virion assembly. It is translated from the downstream ORF of the bicistronic mRNA5 in vitro (Thiel and Siddell, 1995), suggesting that translation of the E protein is cap-independent, possibly via an internal ribosomal entry site (IRES). Subsequently, the IRES has been mapped through a series of deletion mutants in an in vitro translation assay to be located between ≈100-nt upstream and 180-nt downstream of the ORF5b (Jendrach et al., 1999). The gene encoding the E in other coronaviruses includes ORF5b in bovine Coronavirus (BCoV), ORF4 in transmissible gastroenteritis virus (TGEV) and human Coronavirus (HCoV) 229E, and ORF3c in avian infectious bronchititis virus (IBV). Interestingly, while the ORF4 of TGEV and HCoV-229E, and ORF5b of BCoV are translated via the cap-dependent mechanism from a distinct mRNA species, mRNA4 and mRNAS-1, respectively, the 0RF3C of IBV and ORF5b of MHV are translated via an IRES from a downstream ORF of mRNA3 and mRNA5, respectively (see review by Lai and Cavanagh, 1997). More intriguingly, within TGEV strains, the ORF3b of Muller strain is translated from mRNA3 via internal ribosomal entry whereas that of the Purdue strain is translated from mRNA3-l, an mRNA species differing from mRNA3, via cap-dependent mechanism (O’Connor and Brian, 2000). It is not known, however, why different strains of the same TGEV or different coronaviruses evolved such distinct mechanisms in regulating the expression of the same genes.
KeywordsmRNA Species Mouse Hepatitis Virus Transmissible Gastroenteritis Virus Internal Ribosomal Entry Site Sequence Murine Coronavirus
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