THE ROLE OF 2',3'-CYCLIC PHOSPHODIESTERS IN THE BOVINE PANCREATIC RIBONUCLEASE-A CATALYZED CLEAVAGE OF RNA - INTERMEDIATES OR PRODUCTS

There is a considerable degree of ambiguity in the literature regardin g the role of the 2',3'-cyclic phosphodiesters formed during the react ion of RNA cleavage catalysed by ribonuclease. Usually the reaction is considered to take place in two steps: in the first step there is a t ransphosphorylation of the RNA 3',5'-phosphodiester bond broken yieldi ng a 2',3-cyclic phosphodiester which in the second step is hydrolysed to a 3'-nucleotide. Although in many occasions, either explicitly or implicitly, the reaction is treated as taking place sequentially, this is not the case as it has been shown that the 2',3'-phosphodiesters a re actually released to the medium as true products of the reaction an d that no hydrolysis of these cyclic compounds takes place until all t he susceptible 3',5'-phosphodiester bonds have been cyclised. Comparis on of the hydrolysis and alcoholysis of the 2,3'-phosphodiesters catal ysed by RNase A indicates that the hydrolysis reaction has to be consi dered formally as a special case of the transphosphorylation back reac tion in which the R group of the R-OH substrate is just H. It is thus concluded that the 2',3'-cyclic phosphodiesters formed in the ribonucl ease A reaction are true products of the transphosphorylation reaction and not intermediates as usually considered.