A bacterial expression cloning screen reveals tardigrade single-stranded DNA-binding proteins as potent desicco-protectants

Tardigrades are known for their ability to survive remarkable extremes, but relatively little is known about the molecular underpinnings that allow for their extraordinary stress tolerance. We devised an expression cloning approach to screen for novel protectants from desiccation-tolerant tardigrades. We expressed cDNA libraries from the tardigrades Hypsibius exemplaris and Ramazzottius varieornatus in E. coli and subjected the bacteria to desiccation. Sequencing the populations of surviving bacteria revealed enrichment of mitochondrial single-stranded DNA-binding proteins (mtSSBs) from both tardigrade species. Expression of mtSSBs in bacteria improved desiccation survival to similar levels as some of the best protectants known to date. The DNA-binding activity of the oligonucleotide/oligosaccharide binding (OB) fold domain of mtSSB was necessary and sufficient to improve desiccation tolerance. By comparing the efficacy of mtSSBs from multiple organisms, we provide evidence that desicco-protection is a conserved feature of mtSSBs. More broadly, any OB-fold containing protein may act similarly to promote desiccation tolerance, likely by coating ssDNA to limit damage during desiccation. These results identify single-stranded DNA-binding activity as a potent contributor to extreme desiccation tolerance.