Figure 1.

 In vitro processing of a model pre-U5 transcript. (A) Sequence and a secondary structure model of the precursor used in this study. The secondary structure of mature U5 is drawn from Frank et al. (1994). The most stable potential secondary structure of the downstream genomic sequence calculated using Mfold (Zuker 1994) is shown here but has not been proved experimentally nor phylogenetically. Sequences not derived from the U5 gene are indicated in lowercase. They include two additional guanosines at the 5′ end of the molecule to facilitate transcription by T7 RNA polymerase and part of a BamHI restriction site at the 3′ end. (B) Time course of processing. Precursor-U5 (P) was incubated in a whole-cell extract for the times indicated. Shown is an autoradiograph of a 6% gel. σ and λ are generated rapidly and have the characteristics of intermediates, whereas U5L and U5S have characteristics of products (see text). The molecular weight marker (M) is a pBR322 plasmid digested with MspI and labeled with [γ-³²P]ATP.