Icosahedral ssRNA Bacterial Viruses

Abstract

Bacterial viruses with single-stranded RNA genomes – the RNA bacteriophages – have been extensively studied since their discovery in the early 1960s. X-ray crystal structures of a number of individual intact phages (MS2, Qβ, fr, GA, and PP7) and a number of recombinant protein shells have been reported at atomic resolution and cryoelectron microscopy studies for several of the phages have also been completed. The phage capsids are examples of T = 3 quasi-equivalent shells, being composed of 180 copies of a single type of coat protein subunit, packaged as non-covalent dimers, and one or a few copies of a maturation protein(s). Their genomes are positive-sense, polycistronic RNAs that are normally ∼3500–4300 bases long. These encode essentially only four protein products: the coat and maturation proteins, a replicase subunit, and lysis protein. Despite their apparent simplicity, gene expression from the genomic RNA is highly regulated both by dynamic RNA folding, which can sequester ribosome binding sites in an inaccessible state(s), and by a well-characterized coat protein translational repression event. RNA replication is also tightly controlled by the replication system to prevent formation of double-stranded RNAs. Many fundamental studies have made use of these phages as models. For instance, the translational repression complex has been used as a paradigm for understanding the molecular details of sequence-specific RNA–protein interactions, and forms the basis of the three-hybrid assay for detecting molecular interactions in vitro. Other applications of these phages are also beginning to be reported.