Phages are viruses of bacteria and are the smallest and most common biological entities in the environment. They can reproduce immediately after infection or integrate as a prophage into their host genome. SPβ is a prophage of the Gram-positive model organism Bacillus subtilis 168, and it has been known for more than 50 years. It is sensitive to dsDNA damage and is induced through exposure to mitomycin C or UV radiation. When induced from the prophage, SPβ requires 90 min to produce and release about 30 virions. Genomes of sequenced related strains range between 128 and 140 kb, and particle-packed dsDNA exhibits terminal redundancy. Formed particles are of the Siphoviridae morphotype. Related isolates are known to infect other B. subtilis clade members. When infecting a new host, SPβ presumably follows a two-step strategy, adsorbing primarily to teichoic acid and secondarily to a yet unknown factor. Once in the host, SPβ-related phages pass through complex lysis-lysogeny decisions and either enter a lytic cycle or integrate as a dormant prophage. As prophages, SPβ-related phages integrate at the host chromosome's replication terminus, and frequently into the spsM or kamA gene. As a prophage, it imparts additional properties to its host via phage-encoded proteins. The most notable of these functional proteins is sublancin 168, which is used as a molecular weapon by the host and ensures prophage maintenance. In this review, we summarise the existing knowledge about the biology of the phage regarding its life cycle and discuss its potential as a research object.
The Bacillus phage SPβ has been known for about 50 years, but only a few strains are avalible. We isolated four new wild type strains of the SPbeta species. Phage vB_BsuS-Goe14 introduces its prophage into the spoVK locus, previously not observed to be used by SPβ-like phages. We could also reveal the SPβ-like phage genome replication strategy, the genome packaging mode, and the phage genome opening point. We extracted 55 SPβ-like prophages from public Bacillus genomes, thereby discovering three more integration loci and one additional type of integrase. The identified prophages resembled four new species clusters and three species orphans in the genus Spbetavirus. The determined core proteome of all SPβ-like prophages consists of 38 proteins. The integration cassette proved to be not conserved even though present in all strains. It consists of distinct integrases. Analysis of SPβ transcriptomes revealed three conserved genes, yopQ, yopR, and yokI, to be transcribed from a dormant prophage. While yopQ and yokI could be deleted from the prophage without activating the prophage, damaging of yopR led to a clear-plaque phenotype. Under the applied laboratory conditions, the yokI mutant showed an elevated virion release implying the YokI protein being a component of the arbitrium system.