| Single-stranded DNA binding proteins (SSB) play an essential role in all cellular DNA transactions including DNA replication, repair, and recombination. At the replication fork, SSBs serve to stabilize and protect the transient intermediate ssDNA, which is generated by a replicative helicase. Several members of this protein family are reported to stimulate their cognate DNA polymerases during DNA synthesis, as well as promote strand exchange in homologous recombination. In the Archaea, which together with Bacteria and Eukarya constitute the three domains of life, single-stranded DNA binding proteins have been described for only thermophilic and hyperthermophilic organisms, although this domain includes numerous mesophilic genera. As observed with other proteins involved in information processing, SSB or replication protein A in archaea exhibit more similarity to eukaryotic RPAs than to bacterial SSBs. Currently, cultivable archaea are grouped into two subdomains, namely the Crenarchaeota and the Euryarchaeota. While to date the crenarchaeotes have been shown to harbor a single gene encoding a single oligosaccharide/oligonucleotide (OB) binding fold RPA, the euryarchaeotes on the other hand display an unusual diversity of these proteins. The striking homologies among the individual OB folds of the eukaryotic RPAs and the significant structural similarity they share with the ssDNA binding domain of bacterial SSBs have led to the postulate that RPAs evolved from gene duplication events in an ancestral SSB gene. Here, we show that no group of organisms has exploited the potential in the ssDNA-binding module for engineering of novel RPAs as the archaeal subdomain Euryarchaeota have. Members of this group, which includes hyperthermophiles, mesophiles, halophiles, sulfate reducers, acidophiles, and methanogens, have reinvented RPAs many times and in the ensuing research molecular and biochemical approaches are used to study the structure and function of candidate RPAs from three archaeal species of distinct ecological niches. In addition, the evolutionary implications of the existence of this large numbers of RPAs are examined experimentally and theoretically. |
