PHOSPHOROTHIOATES IN PRE-TRANSFER RNAS CAN CHANGE THE SPECIFICITIES OF RNASES-P OR REDUCE THE CLEAVAGE EFFICIENCIES

Several phosphorothioate-modified E coli and yeast pre-tRNAs were synt hesized. If this modification included the phosphodiester at the RNase P cleavage site, two different effects were observed. With some pre-t RNAs the RNase P cleavage efficiency was severely reduced, whereas wit h other pre-tRNAs a new reaction type for RNase P was observed. Unlike the previously studied base or ribose modifications, phosphorothioate s resulted in aberrant cleavages at unmodified phosphodiesters. These new sites could be located in the 5'-flank or in the acceptor stem of the tRNA domain. Modified mutants of E coli pre-tRNA(Tyr) with differe nt base pairs at the RNase P cleavage site were cleaved with reduced e fficiencies, but no aberrant products were observed.