The life cycle of SPβ and related phages

Abstract

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.

Fig. 1

The life cycle of SPβ and related phages

Fig. 2

Virion of the SPβ-like phage Goe12 (vB_BsuS-Goe12)

Fig. 3

Average nucleotide sequence identity of SPβ-related phages. Blue–white (70–95% identity) indicates affiliation to the same genus; and white–red (95–100% identity), to the same species.

Fig. 4

The genome of Bacillus phage SPβ. The genome orientation and the locations of its genomic clusters I–III are defined with respect to phage replication (I, early; II, early; III-late) and adjusted according to Lazarevic et al. [44]. Arrows indicate protein-coding genes. Red arrows represent genes encoding hypothetical proteins not discussed in this review. Purple arrows represent genes discussed in this review to which gene names are connected with a black line. The attP site, the pac site, the SOS boxes, the recombination unit, the sublancin 168 cluster, and the arbitrium system are indicated. The genome map's initial structure was created with Clone Manager 8 (Sci Ed Software, Westminster, Colorado, USA) using the SPβ c2 genome sequence [44] and elaborated further with MS PowerPoint 2019.