In the first step of our study, we undertook the determination of the structural features of the entire short 3 0 UTR of the rat DNA polymerase b transcript (208 nt). For this purpose, we used limited fragmentation of the 5 0 -end-labeled RNA with six well-characterized biochemical structural probes: lead ions and five enzymes ( Figure 1A ). Nuclease S1 and lead ions have no documented nucleotide specificity, while ribonuclease T2 and A recognize all nucleotides but have a higher activity for adenosines and pyrimidines, respectively. RNase T1 exhibits specificity exclusively for guanosines and RNase Cl3 digests C-residues only. All the above structural probes recognize flexible and single-stranded regions in the RNA structure. Prior to the probing experiments, the 5 0 -end labeled transcript of the Pol b short 3 0 UTR was analyzed by non-denaturing gel electrophoresis. The result of this test suggests that only a single conformer is formed by the analyzed RNA molecule (migrated as a single band on a gel, not shown).