In vitro import of a nicked tRK1 transcript. (a) Secondary structures of tRK1 and of the truncated A and B fragments. (b) Nondenaturing gel electrophoresis of fragments A and B: simple mixture of the two fragments (lane A+B), reannealed A+B fragments [lane A+B(hybrid.)], renatured tRK1 transcript (lane tr.N°1). (c) Autoradiographic detection on a denaturing polyacrylamide gel of imported A and B fragments (lanes 3–6) and of the nicked tRK1 transcript (lanes 7 and 8). Before the import reaction, the truncated fragments were aminoacylated with a mixture of yeast synthetases and a mixture of amino acids and the reannealed fragments with KRS and lysine. The amount of labeled A and B fragment used per assay was 6 pmol of fragment A (lanes 1–8) and either 12 pmol (lanes 1–6 and 8) or 3 pmol (lane 7) of fragment B. Control, labeled fragments A (lane 1) and B (lane 2) used as markers; imported RNAs, in vitro import assays of fragment A (lane 3), fragment B (lane 4), fragment B in the presence of either 400 pmol (lane 5) or 800 pmol (lane 6) of unlabeled tRK1 and of the hybridized A+B fragments (lanes 7 and 8). (d) Possible secondary structure of the B fragment.