SAAV2152 is a single-stranded DNA binding protein: the third SSB in Staphylococcus aureus

Single-stranded DNA-binding proteins (SSBs) play crucial roles in DNA replication, repair, and recombination. Unlike E. coli, which contains only one type of SSB (EcSSB), some bacteria have two paralogous SSBs, namely, SsbA and SsbB. In this study, we found the third SSB-like protein in Staphylococcus aureus, SAAV2152, which was designated as SaSsbC. SaSsbC is a protein of 131 amino acids and shares 38%, 36%, and 33% sequence identity to SaSsbB, SaSsbA, and EcSSB, respectively. Gene map analysis showed that unlike the E. coli ssb gene, which is adjacent to uvrA gene, the S. aureus ssb gene SAAV2152 is flanked by the putative SceD, the putative YwpF, and fabZ genes. A homology model showed that SaSsbC consists of the classic oligonucleotide/oligosaccharide-binding fold at the N-terminus. At the C-terminus, SaSsbC did not exhibit sequence similarity to that of EcSSB. Electrophoretic mobility shift analysis showed that SaSsbC formed a single complex with ssDNA of different lengths. Mutational analysis revealed that Tyr36, Tyr47, Phe53, and Tyr81 in SaSsbC are at positions that structurally correspond to the important residues of EcSSB for binding to ssDNA and are also critical for SaSsbC to bind ssDNA. Unlike EcSSB, which can stimulate EcPriA, SaSsbC did not affect the activity of SaPriA. In addition, SaSsbA inhibitor 9-methyl-2,3,7-trihydroxy-6-fluorone (NSC5426) could inhibit the ssDNA-binding activity of SaSsbC with IC(50) of 78 μM. In conclusion, this study has identified and characterized SAAV2152 as a kind of SSB, and further research can directly focus on determining its actual physiological role in S. aureus.