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 beta1-beta2 and beta3-beta4 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 specificity 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
beta3-beta4.
