Arg135 and Asp145 are situated on the opposite face of the β-hairpin, where they form an interstrand salt bridge (Figure 6A). It was noteworthy that whereas subtracting the Asp145 side chain had no apparent impact on cell growth, the loss of Arg135 was lethal. Thus, the Asp-Arg salt bridge is not essential for Rpo26 activity. Arg135 forms a cation-π stack on Phe143 (Figure 6A), and we suspect that this cation-π interaction accounts for the essentiality of Arg135. Consistent with this idea, replacing Arg135 with lysine, which would, in principle, preserve the cation-π interaction, had no effect on yeast growth (Nouraini et al. 1996a).