U27. U27 forms only a single hydrogen bond via O2 to the amino group of G15, putting less structural restraints on frameshifting. Still mutation of this residue reduced frameshifting 1.4-to 2.5-fold ( Table 1 ). The 2-fold reduction seen by substituting C for U27 is not easily explained as this pyrimidine has an identically positioned O 2 to hydrogen bond to the amino group of G15.