MHV belongs to the Coronaviridae family of positivesense, single-stranded RNA viruses. MHV replication and transcription occur exclusively in the cytoplasm of infected cells via the viral RNA-dependent RNA polymerase (RdRp) (reviewed by Lai and Cavanagh, 1997) . Initially, the 5¢-most gene 1 of the viral genome is translated into the viral RdRp, which then replicates the viral genomic RNAs into negative-strand RNAs. Subsequently, the negative-strand RNAs are used as templates to transcribe mRNAs, which include a genomic-sized RNA and a nested set of subgenomic mRNA transcripts, all with an identical 5¢ non-translated leader sequence of 72±77 nucleotides and 3¢ co-terminal polyadenylated ends. The subgenomic mRNA transcription of MHV utilizes a unique discontinuous mechanism in which the leader sequence, often derived from a different molecule, is fused to RNAs at the intergenic (IG) sites (i.e. transcription initiation site) to generate subgenomic mRNAs (Jeong and Makino, 1994; Liao and Lai, 1994; Zhang et al., 1994) . The exact mechanism of how these mRNAs are made is still controversial. However, it has been shown that the process of discontinuous RNA transcription is regulated by several viral RNA elements, including the cis-and trans-acting leader RNA Zhang et al., 1994) , IG sequence (Makino et al., 1991) and 3¢-end untranslated sequence (Lin et al., 1996) . There is considerable biochemical evidence suggesting possible direct or indirect interactions between the various RNA regulatory elements. hnRNP A1 binds MHV negative (±)-strand leader and IG sequences (Furuya and Lai, 1993; Li et al., 1997) . Site-directed mutagenesis of the IG sequences demonstrated that the extent of binding of hnRNP A1 to the IG sequences correlated with the ef®ciency of transcription from the IG site (Zhang and Lai, 1995; Li et al., 1997) . Immunostaining of hnRNP A1 showed that hnRNP A1 relocated to the cytoplasm of MHV-infected cells, where viral RNA synthesis occurs (Li et al., 1997) . hnRNP A1 also mediates the formation of a ribonucleoprotein complex containing the MHV (±)-strand leader and IG sequences . These results suggest that hnRNP A1 may serve as a protein mediator for distant RNA regions to interact with each other. Heterogeneous nuclear ribonucleoprotein A1 regulates RNA synthesis of a cytoplasmic virus The EMBO Journal Vol. 19 No. 17 pp. 4701±4711, 2000 ã European Molecular Biology Organization Many cellular proteins, including calreticulin (Singh et al., 1994) , polypyrimidine tract-binding protein (PTB) (Hellen et al., 1994; Wu-Baer et al., 1996) , La protein (Pardigon and Strauss, 1996), Sam68 (McBride et al., 1996) , poly(rC)-binding protein (Parsley et al., 1997) and nucleolin (Waggoner and Sarnow, 1998) , have been implicated to be involved in viral RNA transcription or replication. In addition to MHV, hnRNP A1 has also been reported to interact with human cytomegalovirus immediate-early gene 2 protein, which plays an important role in the regulation of virus replication (Wang et al., 1997) . Furthermore, a yeast protein related to human core RNA splicing factors, Lsm1p, has been shown to be required for the ef®cient replication of brome mosaic virus RNA (Diez et al., 2000) . Recently, Reddy and colleagues demonstrated an inhibition of HIV replication by dominant-negative mutants of Sam68 (Reddy et al., 1999) . However, none of these cellular proteins has been shown experimentally to participate directly in RNA-dependent RNA synthesis.