Plus-strand RNA viruses have genomes that can act as mRNA and be translated upon entry into the host cell. One of the first products is an RNA-dependent RNA polymerase, which catalyzes the synthesis of negative-strand RNAs; these are then used to make more plus-strand RNAs (figure).
In some cases, this occurs by way of a double-stranded replicative form (RF), just as seen with ssDNA viruses (section). In-plant and animal viruses, viral genome replication occurs in a structure formed within the cytoplasm called a replication complex (see figure).
The plus-strand RNA genome can serve directly as mRN. One of the first viral proteins synthesized is replicase, which replicates the genome, sometimes via a double-stranded replicative form. Transcriptase is responsible for synthesizing additional mRNA molecules.
These are compartments formed in response to factors produced by the virus, and they are derived from the membranes of cell organelles (e.g., ER membrane); the source of the membrane depends on the virus.
Assembly of progeny virions sometimes also occurs within the replication complex. There are a number of important positive-strand animal viruses (e.g., the viruses that cause polio, SARS, and hepatitis A), and most plant viruses have plus-stranded RNA genomes.
Plus-Strand RNA Viruses: Genomes That Can Be Translated upon Entry
■ The genomes of plus-strand RNA viruses serve directly as mRNA molecules. Among the first viral proteins synthesized is an RNA-dependent RNA polymerase that replicates the plus-strand RNA genome, sometimes by forming a dsRNA replicative intermediate.
■ The negative RNA strands produced by the RNA-dependent RNA polymerase can be used to make either more genomes or mRNA (figure).
■ Poliovirus genomic RNA is translated into a single polyprotein that is cleaved to form all the proteins needed by the virus during its life cycle (figures).
■ TMV is like most known plant viruses in that it has a plus-strand RNA genome. Translation of TMV genomic RNA yields two proteins, the larger of which is produced by a readthrough mechanism.
■ The RNA-dependent RNA polymerase encoded by the genome synthesizes minusstrand RNAs that serve as templates for synthesis of subgenomic mRNAs that are translated to yield other proteins needed by the virus. The minus-strand RNAs also serve as templates for synthesis of new plus-strand RNA genomes.
■ The TMV nucleocapsid forms spontaneously when disks of coat protein protomers complex with the RNA (figure).