FIG. 4.

Citrate and Mg²⁺ titrations of the signaling reaction. Total transcription refers to the yield, in arbitrary PhosphorImager units, of the 120-nt cassette from the mutant template. It is called total transcription because both initiation and elongation efficiencies contribute to the yield. Elongation efficiency is defined here as the efficiency of 120- to 261-nt cassette readthrough, in this case when signaling is absent (i.e., the molar ratio of 261- to 120-nt cassette transcription on the mutant template). Signaling is defined as 100% − % wild-type readthrough (i.e., 100 minus the wild type-versus-mutant elongation efficiency ratio, expressed as a percentage). Points without error bars correspond to values taken from a single experiment only. Several different extract preparations were used for the experiments reported in this paper. Therefore, the citrate and Mg²⁺ optima exhibited in this figure differ slightly from the concentrations used for other experiments reflecting the use of a different extract preparation here. Also note in panels B and C that equivalent signaling efficiencies can be obtained using different combinations of citrate and Mg²⁺ concentrations, further accounting for the use of different concentrations of these constituents in different experiments.