In vivo non-specific binding of λ CI and Cro repressors is significant

We propose a thermodynamic model that includes the non-specific binding of the lambda. phage regulatory proteins CI and Cro. By fitting the model to experimental in vivo data on activities of the two promoters P-RM and P-R versus concentration, we estimate the free energy upon non-specific binding to be -4.1 +/- 0.9 kcal/mol for CI and -4.2 +/- 0.8 kcal/mol for Cro. For concentrations > 100 nM of CI or Cro, we find that > 50% of these proteins are non-specifically bound. In particular, in a lysogen (similar to250 CI monomeric equivalents per cell) nearly 90% of CI is non-specifically bound. (C) 2004 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.