Intracellular equilibrium calculations based on small systems thermodynamics

Regulation of many processes in the living cell, such as gene expression and DNA replication, are believed to be controlled by interactions of regulatory molecules with inducers, repressors, nucleic acid sequences, protein domains, and with each other. Mathematical models of these regulatory systems have been proven quite useful for quantitative description of cell population kinetic properties. The purpose of this communication is presentation of a general algorithm for calculation of intracellular interaction equilibria in cell populations based on statistical thermodynamics appropriate to the small number of molecules-intracellular situation. This algorithm is applied to a five-component model of regulation of transcription by the lac promoter-operator, and the results are compared with those based on the traditional single-cell equilibrium approach.