With an excitation wavelength of 295 nm, the emission spectrum of SSB proteins at 25°C had a maximum at 348 nm, which is consistent with tryptophan fluorescence. When adding a saturating quantity of ssDNA, the intrinsic fluorescence at 348 nm was quenched by 95% for both the TmaSSB
and the TneSSB proteins. The estimated size of the ssDNA binding site in the presence of 2 or 100 mM of NaCl for the TmaSSB and the TneSSB proteins was 68 ± 2 nt (Figure 5). None binding-mode transition was observed when changing S63845 chemical structure the ionic strength from low (2 mM NaCl) to high salt (100 mM NaCl). In all cases, the cooperative affinity is estimated to be in the range of 107-108 M-1. Figure 5 Inverse fluorescence titration of Tma SSB and Tne SSB with (dT) 76 . A 1 nM sample of TmaSSB (A) and TneSSB (B) was titrated with (dT)76 at 2 mM NaCl (filled figures) or 100 mM NaCl (open figures) in binding buffer. Thermostability The half-lives of the ssDNA-binding activities of TmaSSB and TneSSB at 100°C, determined by gel mobility shift assays, were 10 h and 12 h, respectively.
The thermostability for TaqSSB was 30 s at 95°C, 3 min at 90°C and 15 min at 85°C, as was also shown Dorsomorphin by Dąbrowski et al. . When analyzed by differential scanning microcalorimetry (DSC) the thermal unfolding of TmaSSB, TneSSB and TaqSSB was found to be an irreversible process, as seen in the rescan thermograms Phosphatidylinositol diacylglycerol-lyase (Figure 6). The TneSSB had the highest thermostability, with a melting temperature (T m) of 112,5°C, whereas TmaSSB had a Tm of 109,3°C (Figure 6). The melting temperature of TaqSSB was only 86,8°C. This difference in T m confirmed the different thermostabilities of the proteins indicated by the observed half-lives of the ssDNA binding activities. The thermograms of these SSB proteins did not
show any characteristic signs of PLX-4720 molecular weight heavily aggregated proteins after heat denaturation. Moreover, the results of the DSC and the half-lives of the ssDNA binding activities suggest that the loss of binding activity of TmaSSB, TneSSB and TaqSSB was connected with an irreversible thermal unfolding of the proteins. Figure 6 DSC thermograms of SSB proteins. Samples containing 1.5 mg/ml SSB were analyzed in 50 mM potassium phosphate buffer pH 7.5 and 0.1 M NaCl. In summary, the results showed that TmaSSB and TneSSB are the most thermostable SSB proteins identified to date. Discussion In this study, we have described the purification and characterization of SSB proteins from the thermophilic bacteria T. maritima and T. neapolitana. The results of the sequence analysis verified that a ssDNA binding domain (the first 106 amino acid residues) in one monomer of both TmaSSB and TneSSB proteins possess a canonical oligonucleotide binding fold (OB-fold), very similar to the observed in the structure of E. coli SSB [23, 24].