Displacement of the canonical single-stranded DNA-binding protein in the Thermoproteales

Single-stranded DNA binding proteins (SSBs) based on the OB-fold are considered ubiquitous in nature and play a central role in many DNA transactions including replication, recombination and repair. We demonstrate that the thermoproteales, a clade of hyperthermophilic crenarchaea, lack a canonical SSB. Instead, they encode a distinct ssDNA-binding protein that we term "ThermoDBP", exemplified by protein Ttx1576 from Thermoproteus tenax. ThermoDBP binds specifically to ssDNA with low sequence specificity. The crystal structure of Ttx1576 reveals a unique fold and mechanism for ssDNA binding, consisting of an extended cleft lined with hydrophobic phenylalanine residues and flanked by basic amino acids. Two ssDNA-binding domains are linked by a coiled-coil leucine zipper. ThermoDBP appears to have displaced the canonical SSB during the diversification of the thermoproteales – a highly unusual example where a “ubiquitous” protein has been lost in evolution.