Microviridae

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.