Inhibitory effect of prophage SPβ fragments on phage SP10 ribonucleotide reductase function and its multiplication in Bacillus subtilis

Suppression of abnormal morphology and extracytoplasmic function sigma activity in Bacillus subtilis ugtP mutant cells by expression of heterologous glucolipid synthases from Acholeplasma laidlawii

Glucolipids in Bacillus subtilis are synthesized by UgtP processively transferring glucose from UDP-glucose to diacylglycerol. Here we conclude that the abnormal morphology of a ugtP mutant is caused by lack of glucolipids, since the same morphology arises after abolition of glucolipid production by disruption of pgcA and gtaB, which are involved in UDP-glucose synthesis. Conversely, expression of a monoglucosyldiacylglycerol (MGlcDG) produced by 1,2-diacylglycerol 3-glucosyltransferase from Acholeplasma laidlawii (alMGS) almost completely suppressed the ugtP disruptant phenotype. Activation of extracytoplasmic function (ECF) sigmas (SigM, SigV, and SigX) in the ugtP mutant was decreased by alMGS expression, and was suppressed to low levels by MgSO₄ addition. When alMGS and alDGS (A. laidlawii 1,2-diacylglycerol-3-glucose (1-2)-glucosyltransferase producing diglucosyldiacylglycerol (DGlcDG)) were simultaneously expressed, SigX activation was repressed to wild type level. These observations suggest that MGlcDG molecules are required for maintenance of B. subtilis cell shape and regulation of ECF sigmas, and DGlcDG regulates SigX activity.

Developmentally-regulated excision of the SPβ prophage reconstitutes a gene required for spore envelope maturation in Bacillus subtilis

Temperate phages infect bacteria by injecting their DNA into bacterial cells, where it becomes incorporated into the host genome as a prophage. In the genome of Bacillus subtilis 168, an active prophage, SPβ, is inserted into a polysaccharide synthesis gene, spsM. Here, we show that a rearrangement occurs during sporulation to reconstitute a functional composite spsM gene by precise excision of SPβ from the chromosome. SPβ excision requires a putative site-specific recombinase, SprA, and an accessory protein, SprB. A minimized SPβ, where all the SPβ genes were deleted, except sprA and sprB, retained the SPβ excision activity during sporulation, demonstrating that sprA and sprB are necessary and sufficient for the excision. While expression of sprA was observed during vegetative growth, sprB was induced during sporulation and upon mitomycin C treatment, which triggers the phage lytic cycle. We also demonstrated that overexpression of sprB (but not of sprA) resulted in SPβ prophage excision without triggering the lytic cycle. These results suggest that sprB is the factor that controls the timing of phage excision. Furthermore, we provide evidence that spsM is essential for the addition of polysaccharides to the spore envelope. The presence of polysaccharides on the spore surface renders the spore hydrophilic in water. This property may be beneficial in allowing spores to disperse in natural environments via water flow. A similar rearrangement occurs in Bacillus amyloliquefaciens FZB42, where a SPβ-like element is excised during sporulation to reconstitute a polysaccharide synthesis gene, suggesting that this type of gene rearrangement is common in spore-forming bacteria because it can be spread by phage infection.

Integration of prophages into protein-coding sequences of the host chromosome generally results in loss of function of the interrupted gene. In the endospore-forming organism Bacillus subtilis strain 168, the SPβ prophage is inserted into a previously-uncharacterized spore polysaccharide synthesis gene, spsM. In vegetative cells, the lytic cycle is induced in response to DNA damage. In the process, SPβ is excised from the genome to form phage particles. Here, we demonstrate that SPβ excision is also a developmentally-regulated event that occurs systematically during sporulation to reconstitute a functional spsM gene. Following asymmetric division of the sporulating cell, two cellular compartments are generated, the forespore, which will mature into a spore, and the mother cell, which is essential to the process of spore maturation. Because phage excision is limited to the mother cell genome, and does not occur in the forespore genome, SPβ is an integral part of the spore genome. Thus, after the spores germinate, the vegetative cells resume growth and the SPβ prophage is propagated vertically to the progeny along with the rest of the host genome. Our results suggest that the two pathways of SPβ excision support both the phage life cycle and normal sporulation of the host cells.

AbpA and AbpB provide anti-phage activity in Escherichia coli

Bacteria have a variety of resistance mechanisms for surviving bacteriophage infections. Here, we describe a novel anti-phage mechanism in Escherichia coli. Cells harboring a plasmid with the genes abpA and abpB, formerly yfjL and yfjK, blocked the propagation of bacteriophages belonging to three families: T4, T2, T7 and λ phages. Both genes were necessary for the inhibition of phage propagation, and deletion of either chromosomal gene resulted in a 20% increase of progeny compared to wild-type cells. Neither overexpression nor deficiency of AbpA and AbpB had any apparent effect on E. coli growth. We isolated seven suppressor mutants of T4 phage that grew weakly on cells overexpressing AbpA and AbpB, and found that their mutations were all located in gene 41, which encodes a replicative DNA helicase that is essential for DNA replication. Furthermore, we demonstrated that AbpA and AbpB inhibited DNA replication and late gene expression of T4 phage. Similarly, DNA replication of T7 and λ phages was also inhibited by AbpA and AbpB. These results strongly suggest that E. coli AbpA and AbpB target DNA replication of phages to block their propagation.

SP10 infectivity is aborted after bacteriophage SP10 infection induces nonA transcription on the prophage SPβ region of the Bacillus subtilis genome

Bacteria have developed various strategies for phage resistance. Infection with phage induces the transcription of part of the phage resistance gene, but the regulatory mechanisms of such transcription remain largely unknown. The phage resistance gene nonA is located on the SPβ prophage region of the Bacillus subtilis Marburg strain genome. The nonA transcript was detected at the late stage of SP10 infection but is undetectable in noninfected cells. The nonA transcript was detected after the induction of the sigma factor Orf199-Orf200 (σ(Orf199-200)), when sigma factors encoded in the SP10 genome were expressed from a xylose-inducible plasmid. Thus, the SP10 sigma factor is an activator of a set of SP10 genes and nonA. The nonA gene encodes a 72-amino-acid protein with a transmembrane motif and has no significant homology with any protein in any database. NonA overexpression halted cell growth and reduced the efficiency of B. subtilis colony formation and respiration activity. In addition, SP10 virion protein synthesis was inhibited in the nonA(+) strain, and SP10 virion particles were scarce in it. These results indicate that NonA is a novel protein that can abort SP10 infection, and its transcription was regulated by SP10 sigma factor.