In the area of genetically engineered micro-organisms grown in bioreactors, mathematical modeling usually results in balance type models involving (i) a (rather) large number of state variables and, (ii) complicated kinetic expressions containing a large number of parameters. Therefore, a generic methodology is developed to reduce the model complexity at the level of the kinetics, while maintaining high prediction power. As a case study to illustrate the method and results obtained, the influence of the dissolved oxygen concentration on the cytN gene expression in the bacterium Azospirillum brasilense Sp7 is modeled.