The canonical single-stranded DNA binding protein SSB is not an essential replication protein but an RNA chaperon in the hyperthermophilic archaeonSaccharolobus islandicusREY15A

Abstract

AbstractSingle-stranded DNA binding proteins (SSBs) have been regarded as indispensable factors in all three domains of life since they play vital roles in DNA replication. Herein, we report that genes coding for the canonical SSB (SisSSB) and the non-canonical SSB (SisDBP) in the hyperthermophilic archaeonSaccharolobus islandicusREY15A can both be deleted. The growth is not affected, and the cell cycle progression and genome stability of the deletion strains is not impaired, suggesting that SisSSB and SisDBP are not essential for cell viability. Interestingly, at a lower temperature (55°C), the protein level of SisSSB increases ∼1.8 fold in the wild type and the growth of ΔSisssband ΔSisssbΔSisdbpis retarded. SisSSB exhibits melting activity on dsRNA and DNA/RNA hybridin vitroand unwinding RNA hairpin inEscherichia coli. Furthermore, the core SisSSB domain is able to complement the absence of the cold shock proteins CspABGE inE. coli, suggesting that SisSSB functions as RNA chaperon. We show that a two-fold overexpression of SisSSB is beneficial to the cell growth at lower temperature, but it has detrimental effect on the cell growth and cell cycle progression at normal growth temperature, which differs from bacterial Csp proteins. Importantly, thesein vitroandin vivoactivities are conserved in SSB subtype SSB-1 in Crenarchaeota species that lack bacterial Csp homologs. Overall, we have clarified the function of the archaeal canonical SSB which does not function as a DNA processing factor, but plays a role in processes requiring dsRNA or DNA/RNA hybrid unwinding.