SsbA

SsbA is a single-stranded DNA-binding protein found in many Gram-positive bacteria, notably Bacillus species. It is one of two paralogous SSB proteins in some organisms, alongside SsbB; while SsbB is often induced during competence for DNA uptake, SsbA is typically expressed constitutively and participates in essential DNA metabolism functions such as replication and recombinational repair.

SsbA binds exposed single-stranded DNA with high affinity, stabilizing ssDNA and preventing secondary structure formation. Most SsbA proteins contain a conserved oligonucleotide/oligosaccharide-binding (OB) fold for DNA binding, and an acidic C-terminal tail that mediates interactions with partner proteins such as RecA, PriA, and other components of the replication fork or recombination machinery. The oligomeric state can vary by species, but SSB proteins commonly form multimeric complexes on ssDNA.

In Bacillus subtilis, SsbA contributes to initiation and progression of DNA replication and promotes RecA-mediated strand exchange; it also participates in DNA repair pathways and replication restart after fork stalling. SsbB, by contrast, is primarily associated with competence for natural transformation and exhibits different regulation and function. The presence of SsbA and SsbB as paralogs reflects functional specialization of SSB proteins in Gram-positive bacteria.

Gene context and regulation: ssbA genes are usually encoded separately from ssbB, with expression maintained under normal growth conditions, ensuring baseline SSB activity for genome maintenance. Across species, SsbA proteins preserve the core characteristics of SSBs while adapting to organism-specific replication and repair requirements.