Subsequently, mSP-SL-HIV-1 mRNA was probed in complex with the ribosome and tRNA. For each chemical probe tested, bands that were present in a control lane of unmodified mRNA incubated with ribosome were not taken into account. In the complex, the characteristic strong protections at the guanine nucleotides in the Shine-Dalgarno sequence (G -11 , G -10 ) are clearly seen. Base A +1 , of the methionine codon is protected from chemical modifications demonstrating, as previously described (56), Watson-Crick pairing with the anticodon of tRNA fMet . Interestingly, nucleotide A +6 experienced an increase in reactivity similar to what has previously been seen for a mRNA containing a hairpin (selenocysteine incorporation sequence SECIS) in complex with 30 S subunit and tRNA fMet (57) . In the spacer and hairpin regions, most of the changes in chemical reactivities were detected in the lower stem that is supposed to be close to the ribosomal surface. Bases G +7 , G +8 and G +12 were protected from modification by KE (Figure 4) . The reactivity of G +9 toward KE slightly increased in a way similar as A +10 toward DMS.