The À1 ribosomal frameshifting events were monitored by the SF reporter construct described earlier (27) . Complementary oligonucleotides (Eurogentec, Liege, Belgium) SF462 (CTAGTTGACCTCAACCCTTGG AA) and SF463 (CATGTTCCAAGGGTTGAGGT CAA) and SF468 (CTAGTTGAGCGCGCTGGAGGC CATGG) and SF469 (CATGCCATGGCCTCCAGCGC GCTCA) were annealed and ligated into SpeI/NcoI digested SF reporter to construct the SF462 and SF468 templates, respectively. All constructs were verified by DNA sequencing on an ABI PRISM Õ 3730xl analyzer (LGTC, Leiden, The Netherlands). RNA oligonucleotides (except for RNA13 which was obtained from Invitrogen) were purchased from Dharmacon (Lafayette, USA). The RNAs from Dharmacon carried a 2 0 -O-ACE protection group, which was removed by incubation with 100 mM acetic acid pH 3.8 and TEMED at 60 C for 30 min. The sequences of RNA oligos were as follows: