Figure 5.

Effects of alanine substitutions at positions 192 and 197 on interaction of λcI CTDs. Depicted above the graph is the reporter construct present in strain KS1. Interaction between the CTDs of the DNA-bound λcI dimer and the CTDs fused to the α-NTD results in transcriptional activation from this reporter. The graph shows the results of β-galactosidase assays performed on cultures of KS1 cells containing plasmids pACλcI and pBRα–cI (○), pACλcI and pBRα–cI–D197A (▾), or pACλcI–K192A and pBRα–cI (•) grown in medium containing IPTG at concentrations of 0, 5, and 20 μm. Graph labels list the λcI allele first and the α–cI allele second. Previous experiments with this system have shown that maximum β-galactosidase values are achieved at IPTG concentrations between 10 and 20 μm (Dove et al. 1997). Control experiments indicate that, at these IPTG concentrations, the λ operator is saturated and the fraction of RNAP molecules containing the α–cI chimera approaches a maximum (S.L. Dove and A. Hochschild, unpubl.).