Microviridae

Abstract Members of the Microviridae comprise at least two subfamilies ( Bullavirinae and Gokushovirinae ), with divergent sequences from many uncultured representatives yet to be formally classified. Bullaviruses (canonical species φX174), which infect free‐living bacteria, are among the fastest known replicating viruses. Gokushoviruses were originally thought to occupy a unique niche, infecting obligate intracellular bacteria; however, genomic analyses suggest that this group infects free‐living hosts as well. Some gokushoviruses, unlike other members of the family, can undergo both lytic and lysogenic replication cycles. Microviridae contain small (4000–6000 bases), circular and single‐stranded deoxyribonucleic acid (ssDNA) genomes of positive polarity, which are packaged inside small (∼25 nm diameter) T = 1 icosahedral capsids. The most well‐known member of the Microviridae , φX174, has been fundamental in uncovering the mechanisms of DNA replication and capsid assembly and become a model system for experimental evolution. In contrast, little is known about the replication, structure and host range of gokushoviruses despite viromics indicating their ubiquity throughout the biosphere. Whilst overlapping reading frames increase the amount of genetic information encoded in small genomes, they do not appear to significantly impact the ability of the virus to genetically adapt to selective pressures. Due to the genome's positive polarity, DNA replication must commence before viral genes can be transcribed. Microvirus DNA replication occurs in three distinct stages: (1) ssDNA is first converted to a double‐stranded molecule, (2) amplification of the double‐stranded molecule and (3) single‐stranded genomic DNA synthesis and packaging. Genomic DNA synthesis and packaging are concurrent processes; thus, a genome is not synthesised unless there exists a capsid in which to package it. Gene expression is controlled by the finely tuned interplay of cis ‐acting genetic elements: promoters, ribosome‐binding sites, mRNA stability sequences and transcription terminators. Bullaviruses (φX174, G4 and α3) are distinguished by their two‐scaffolding protein system, whereas gokushoviruses utilise a single scaffolding protein. Scaffolding proteins induce conformational switches in the viral coat protein to control the timing and fidelity of morphogenesis. Cell lysis is achieved by inhibiting host cell‐wall biosynthesis, a mechanism reminiscent of some antibiotics. Viromics has revealed the ubiquity of Microviridae genomes throughout the biosphere, with gokushoviruses and uncharacterised divergent lineages being the most common. Hosts are unknown for the vast majority of environmental Microviridae , leading to a dearth of knowledge on their gene functions, replication strategies, virion structures or even absolute abundance in the environment. Although the Microviridae were traditionally thought to be purely lytic, genomic studies have demonstrated that some gokushoviruses are capable of lysogeny.