FIG. 4.

Kinetic analysis of the recruitment of small ribosomal subunits. Shortened transcripts of IRE.34 (A to C), IRE.66 (D to F), and IRE.100 (G to I) were assayed in the presence of either 45 ng of IRE competitor mRNA (open squares) or 125 ng of recombinant IRP-1 (solid diamonds) for 5 (A, D, and G), 15 (B, E, and H), or 30 (C, F, and I) min. The assays contained 0.5 mM GMP-PNP and were analyzed by using 5 to 25% linear sucrose gradients. The labelled mRNA in the fractions is expressed as a percentage of total counts recovered and is plotted against the fraction number. The dashed line denotes the A₂₅₄ profiles, which were identical for gradients with added IRE or IRP-1.