The binding affinity of GC33 and TcaR was determined quantitatively using surface plasmon resonance. Increasing concentrations of TcaR were passed across a flow cell coated with ssDNA GC33 and the binding response was recorded as changes in response units (RU) after subtraction of the binding response for the reference flow cell. Figure 2A shows a representative sensorgram. Analysis of the sensorgram data indicates k a for the interaction of TcaR with the ssDNA GC33 is 8.8610 5 M 21 s 21 ; k d for the interaction of TcaR with the ssDNA GC33 is 9.5610 23 s 21 (Table 1) . To investigate whether TcaR binds to different types and different lengths of DNA molecules, Biacore experiment was used to test the binding of TcaR to DNA fragments of biotin-labeled ssDNA and hairpin DNA duplex (IcaR DNA1). Consistent with previous observations in Fig. 1B , TcaR binds to ssDNA GC33 and ssDNA AT33 with the higher affinity and the association rates of TcaR to IcaR DNA1, and ssDNA GC17, ranging from 4.3610 5 M 21 s 21 to 1.2610 5 M 21 s 21 , are much lower (shown in Figure 2B ). However, when look at a wider range of DNA sizes, TcaR showed no detectable binding ability to ssDNA 8-mer GC8 but the highest binding ability to ssDNA GC99. The association rate with ssDNA GC99 is 36-fold higher than with ssDNA GC33, along with a 20-fold higher off-rate, suggested that cooperative binding of TcaR may contribute significantly in its ssDNA binding activity (Table 1) . Interestingly, the association rate of GC33 ssDNA is two-times higher than IcaR DNA1, but the dissociation rate of IcaR DNA1 is the lowest compared to the DNA fragments tested in this study, suggesting different modes of interaction occurs in ssDNA-TcaR and dsDNA-TcaR complex ( Figure 2B ).