The 208 bp of Pol b 3 0 UTR, encompassing the whole short 3 0 UTR sequence with the hairpin encoding region, was cloned into pGEM-3Z and pGEM-4Z vectors (Promega) in both orientations, generating a set of templates for transcription of sense and antisense mRNAs. In vitro transcription was carried out with polymerases SP6 and T7 (Promega). [a-32 P]UTP (3000 Ci/mmol) was used for labeling. RNA integrity was controlled by polyacrylamide gel electrophoresis. Labeled transcripts were gel-purified (5% polyacrylamide, 8 M urea).