Identification and characterization of single-stranded-DNA-binding proteins from Thermus thermophilus and Thermus aquaticus - new arrangement of binding domains.

Single-stranded-DNA-binding proteins (SSBs) play essential roles in DNA replication, recombination and repair in bacteria, archaea and eukarya. This paper reports the identification and characterization of the SSB-like proteins of the thermophilic bacteria Thermus thermophilus and Thermus aquaticus. These proteins (TthSSB and TaqSSB), in contrast to their known counterparts from mesophilic bacteria, archaea and eukarya, are homodimers, and each monomer contains two ssDNA-binding domains with a conserved OB (oligonucleotide/oligosaccharide-binding) fold, as deduced from the sequence analysis. The N-terminal domain is located in the region from amino acid 1 to 123 and the C-terminal domain is located between amino acids 124 and 264 or 266 in TthSSB and TaqSSB, respectively. Purified TthSSB or TaqSSB binds only to ssDNA and with high affinity. The binding site size for TaqSSB and TthSSB protein corresponds to 30-35 nucleotides. It is concluded that the SSBs of thermophilic and mesophilic bacteria, archaea and eukarya share a common core ssDNA-binding domain. This ssDNA-binding domain was presumably present in the common ancestor to all three major branches of life.