Figure 1.

Summary of coronavirus RNA synthesis and genetics. (A) A schematic of the gene organization of the MHV genome is shown at the top. Intergenic sequences (IGSs) are marked by arrowheads. Beneath this are represented the positive-stranded genomic RNA (gRNA) and subgenomic RNA species (sgRNA2–7), as well as their negative-strand counterparts, all of which are synthesized in the cytoplasm during the course of infection. Leader and antileader sequences are indicated by ■ and □, respectively. (B) A model for MHV discontinuous transcription mediated by looping out of the template. In this scheme, fusion of the leader to the body of an sgRNA occurs via quasicontinuous synthesis across a junction created by dimerization of proteins bound to identical sequences found at the end of the leader template and at the IGS. (C) Construction of site-specific mutations in the MHV genome by targeted RNA recombination. Mutations (★) are transduced into the MHV genome via in vivo recombination between a synthetic donor RNA and the genome of a recipient virus, the N gene deletion mutant Alb4, which can be selected against on the basis of its thermolability. UTR, untranslated region.