We have investigated the mechanism of overproduction of the multifunctional protein catalyzing the first three steps of UMP biosynthesis in stable mutants of Syrian hamster cells in culture. The rate of degradation of this protein is unaltered in one mutant cell line which overproduces it by 118-fold. In all mutants tested, the increase in the rate of synthesis of this protein is equal to the increase in its steady state concentration. There is a similar correlation between steady state levels of this protein in vivo and the capacity of polysomal RNA isolated from these cells to direct the synthesis of the protein in vitro. The one mutant cell line studied contains large amounts of a polysomal poly(A)-containing RNA (Mr = 2.7 +/- 0.2 times 10(6)) that is not detected in wild type cells. This large RNA co-sediments in sucrose gradients with the capacity to direct the synthesis of the multifunctional protein in vitro.