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

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Abstract
We propose a thermodynamic model that includes the non‐specific binding of the λ 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 (∼250 CI monomeric equivalents per cell) nearly 90% of CI is non‐specifically bound.