Recognition of T-rich single-stranded DNA by the cold shock protein Bs-CspB in solution

Cold shock proteins (CSP) belong to the family of single-stranded nucleic acid binding proteins with OB-fold. CSP are believed to function as ‘RNA chaperones ’ and during anti-termination. We determined the solution structure of Bs-CspB bound to the single-stranded DNA (ssDNA) fragment heptathymidine (dT7) by NMR spectroscopy. Bs-CspB reveals an almost invariant conformation when bound to dT7 with only minor reorientations in loop b1–b2 and b3–b4 and of few aromatic side chains involved in base stacking. Binding studies of protein variants and mutated ssDNA demonstrated that Bs-CspB associates with ssDNA at almost diffusion controlled rates and low sequence speci-ficity consistent with its biological function. A variation of the ssDNA affinity is accomplished solely by changes of the dissociation rate. 15N NMR relaxation and H/D exchange experiments revealed that binding of dT7 increases the stability of Bs-CspB and reduces the sub-nanosecond dynamics of the entire protein and especially of loop b3–b4