FIG. 6.

Effect of U2 snRNA structure on fragility of artificial U2 tandem arrays. (A) Secondary structure of altered U2 snRNAs. The expanded Sm site AUUUUUUUUG between U2 stem-loops IIb and III is shown explicitly. Green, normal U2 sequence; red, artificial sequence, with stem length in base pairs, and tetraloops for stability. (B) Fragility of artificial U2 arrays. Note that the observed level of fragility varies from one independently derived line to another, presumably reflecting position effects, copy number, and/or the relative (and heritable) activity of resident and artificial arrays (3, 68). Tandem U2 gene copy numbers are in parentheses.