Genome Packaging in Bacterial Viruses

The assembly of mature, infectious progeny virions is the virus life cycle. In order to assemble virions within the host bacterial cell, bacteriophages, like other viruses, must bring together the structural components of the capsid and the chromosomal components of the genome. While in some cases this takes place using a mechanism of co-assembly in which the capsid is assembled around the chromosome, many phages translocate, or package, their respective chromosomes from the cytosol into a preformed capsid precursor. This packaging event has been described in phages with chromosomes comprised of double-stranded DNA (dsDNA) (families Podo-, Sipho-, Myo-, and Tectiviridae), single-stranded DNA (ssDNA) (family Microviridae), and double-stranded RNA (dsRNA) (family Cystoviridae). In the dsDNA phages, this process is driven by a motor complex that binds a unique vertex of the icosahedral procapsid and packages DNA by a force-generating, ATP-driven mechanism. In the ssDNA phages, DNA translocation is coupled to genome replication and involves DNA-binding proteins that assist in moving the chromosome into the capsid. dsRNA phages selectively package multiple single-stranded RNA chromosomes into a capsid using a motor complex at a unique vertex prior to replication inside the capsid. The diversity, fidelity and efficiency of these strategies are summarized in this article.