Based on the known molecular interactions of aporepressor, corepressor, and inducer, a mathematical model of product formation about the Trp operon regulated recombinant microbes has been proposed. The linear relation of the overall transcription rate constant and overall translation rate constant on the specific growth rate has been suggested. We used experimental data where engineered microorganisms express interferon (IFN). Decay constant of IFN, mRNA transcription rate constant, and IFN expression rate constant have been estimated. An equation related to IFN concentration in fermentation on specific growth rate and Trp concentration has been obtained. By application of the equation, the expression of IFN is calculated based on Trp concentration and specific growth rate in fermentation. Also, the optimum overall culture time can be determined.