ZpdN, a Plasmid-Encoded Sigma Factor Homolog, Induces pBS32-Dependent Cell Death in Bacillus subtilis

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• The ancestral Bacillus subtilis strain 3610 harbors the 84-kb plasmid pBS32, often lost in laboratory strains.
• Extrachromosomal plasmids can harbor prophages, which are integrated phage genomes.
• Prophage activation can lead to host cell death.

Purpose of the Study:

• To investigate the response of the Bacillus subtilis plasmid pBS32 to DNA-damaging agents.
• To elucidate the role of pBS32-encoded genes in host cell death and plasmid replication.
• To characterize the nature of the particles released during pBS32-mediated cell death.

Main Methods:

• Treatment of Bacillus subtilis with mitomycin C (a DNA-damaging agent).
• Analysis of plasmid copy number and gene expression.
• Investigation of prophage induction, cell lysis, and particle formation using techniques like CsCl sedimentation.

Main Results:

• Mitomycin C treatment induced a nearly 100-fold increase in pBS32 copy number.
• pBS32-borne prophage genes were essential for mitomycin C-induced cell lysis.
• The sigma factor homolog ZpdN, encoded by pBS32, was necessary and sufficient for plasmid hyperreplication and cell death.
• Plasmid DNA was packaged into defective, unstable phage-like particles, indicated by CsCl sedimentation but not electron microscopy.

Conclusions:

• The Bacillus subtilis plasmid pBS32 encodes a prophage that, upon activation by DNA damage, induces host cell death and releases unstable phage-like particles.
• The ZpdN sigma factor homolog plays a critical role in mediating the plasmid's response to DNA damage.
• The plasmid's entire sequence may be part of the prophage, potentially linking competence inhibition (via ComI) to lysogeny.