Guanosine binds to the Tetrahymena ribozyme in more than one step, and its 2-OH and the nonbridging pro-S(p) phosphoryl oxygen at the cleavage site are required for productive docking

The dynamics of binding of various guanosine, or G, substrates to the Tetrahymena thermophila L-21 ScaI ribozyme have been investigated by fluorescence-detected stopped-flow experiments. Upon rapid mixing of various G substrates with a preformed complex of the ribozyme and the fluorescent 5' splice site analogue CCUCUξA, fluorescence transients that provide rates for binding of G substrates and the rate-limiting step for transesterification are observed. The measured apparent bimolecular rate constant for binding of pG is 103 M-1 s-1, much slower than expected for diffusion. pG appears to bind to the preformed complex of the ribozyme and CCUCUξA in at least two steps, a bimolecular step followed by at least one conformational change. This two-step binding of pG, involving a rapid pre-equilibrium, leads to the slow apparent rate constant for binding of pG. Furthermore, the 2'-OH of pG and of the 3' terminal G of the G substrate GUCG and the nonbridging pro-Sp phosphoryl oxygen atom at the site of phosphoryl transfer on CCUCUξA appear to mediate formation of a properly conformed docked ternary complex of the G substrate, 5' splice site, and ribozyme which may represent an intermediate required for initiation of transesterification. It is possible that the 2'- OH of pG and this nonbridging pro-Sp phosphoryl oxygen interact, directly or indirectly, with one another.