Figure 3.

Mutability of residues based on the results of in vivo selection. The negative bars (in green) represent conserved positions, whereas the positive bars (in red) represent the mutable position. Mutable and conserved positions play negative and positive roles in exon inclusion, respectively. The values of −1 and +17.6 represent the absolutely conserved and the least conserved residues, respectively. The dotted horizontal lines show the cutoff points with the mutability values of +0.2 and −0.2, corresponding to the mutable and the conserved residues, respectively. Based on the stretches of the mutable and the conserved residues, two negative elements (Exinct and 3′-Cluster) and one positive element (Conserved tract) have been shown. Nature of these elements was confirmed by site-specific mutagenesis (52, 70). The exceptionally high mutability of position 54 is consistent with the dominant effect of A54G substitution on exon 7 inclusion (52). Interestingly, 3′-Cluster overlaps with TSL2, a terminal stem-loop structure that was recently found to play an inhibitory role (16; Figure 4).