We describe in this paper the global dynamical behavior of a mathematical model of expression of polymerase in bacteria. This model is given by a differential system and algebraic equations. We use some tools from monotone systems theory with concavity of nonlinearities to obtain a global qualitative result: either the trivial equilibrium is globally stable, either there exists a unique positive equilibrium which is globally stable in the positive orthant. The same result holds for a class of qualitatively defined functions. Some generalizations of this result are given.