RADIATION ENERGY-TRANSFER IN RNA POLYMERS

Ribozymes are a special class of polyribonucleotide (RNA) molecules wh ich possess intrinsic catalytic activity, capable of cleaving nucleic acid substrates. RNA molecules were synthesized containing a hammerhea d ribozyme moiety of 52 nucleotides linked to an inactive leader seque nce, for total lengths of either 262 or 1226 nucleotides. These RNAs w ere frozen and irradiated with high energy electrons. Surviving ribozy me activity was determined, using the ability of the irradiated ribozy mes to cleave a labeled substrate. From the same irradiated samples, t he amount of intact RNA remaining was determined following denaturing gel electrophoresis. Radiation target analyses of these data revealed a structural target size of 80 kDa and a ribozyme activity target size of 15 kDa for the smaller ribozyme, and 319 and 16 kDa, respectively, for the larger ribozyme. The disparity in target size for activity vs structure indicates that, in contrast to proteins, there is no spread of radiation damage far from the primary site of ionization in RNA mo lecules. The smaller target size for activity indicates that only prim ary ionizations occurring in the specific active region are effective. This is similar to the case for oligosaccharides. it is concluded tha t the presence of the ribose sugar in the polymer chain restricts radi ation damage to a small region and prevents major energy transfer thro ughout the molecule.