Figure 1.

(A Upper) Schematic structure of the btuB regulatory region. The positions of regulatory elements are indicated relative to the transcription start site and the start of the btuB coding sequence. The region from +1 to +315 is carried on the DNA template for synthesis of btuB RNA, with transcription from the T7 φ10 late promoter; the region for primer hybridization in the toe-print assay is indicated (Right). (Lower) The major primer extension products are identified by the nucleotide position of their 3′ end. (B Lower) Ribosome binding to lac and btuB RNA. RNA containing the translation initiation region from lacZ and btuB genes were synthesized by using T7 RNA polymerase and used in the primer extension inhibition assay. Incubation in the presence of tRNA^fMet, 20 nM 30S ribosomal subunits, or 5 μM Ado-Cbl, as indicated (Upper), was for 10 min; primer extension was then carried out for 10 min after addition of AMV reverse transcriptase. The four lanes (Left) (A,C,G,T) are sequencing ladders for the btuB region; (Right) the positions of the last base of the primer extension fragments.