Identification and properties of the crenarchaeal single-stranded DNA binding protein from Sulfolobus solfataricus

Single-stranded DNA binding proteins (SSBs) play central roles in cellular and viral processes involving the:generation of single-stranded DNA, These include DNA replication, homologous recombination and DNA repair pathways. SSBs bind DNA using four 'OB-fold' (oligonucleotide/oligosaccharide binding fold) domains that can be organised in a variety of overall quaternary str uctures. Thus eubacterial SSBs are homotetrameric whilst the eucaryal RPA p rotein is a-heterotrimer and euryarchaeal proteins vary significantly in th eir subunit compositions, We demonstrate that the crenarchaeal SSB protein is an abundant protein with a unique structural organisation, existing as a monomer in solution and multimerising on DNA binding. The protein binds si ngle-stranded DMA distributively with a binding site size of similar to5 nt per monomer, Sulfolobus SSB lacks the zinc finger motif found in the eucar yal and euryarchaeal proteins, possessing instead a flexible C-terminal tai l, sensitive to trypsin digestion, that is not required for DNA binding, In comparison with Escherichia coli SSB, the tail may play a role in protein- protein interactions during DNA replication and repair.