Kinetics of expression of inducible beta-galactosidase in murine fibroblasts: high initial rate compared to steady-state expression.

We have constructed a murine fibroblast cell line in which synthesis of beta-galactosidase can be induced by incubation with isopropyl-beta-D-thiogalactopyranoside (IPTG). This was obtained by transfection by both a plasmid expressing lacI and a second plasmid expressing lacZ from a modified simian virus 40 (SV40) promoter containing a lac operator. We have measured the induction kinetics as well as the basal and induced differential rate of synthesis of beta-galactosidase. The steady-state rate of synthesis is tenfold higher in the presence than in the absence of inducer; we calculate an average of 1200 lacZ polypeptides are synthesized per minute per cell in the induced cultures. However, immediately after induction, the rate of accumulation of beta-galactosidase is up to 50-fold higher than the basal level. Based on our measurements of stability of beta-galactosidase, we suggest that induction may result in a subsequent down-modulation of the transcriptional activity from the induced gene. We hypothesize this inhibition may result from structural changes in DNA components, such as nucleosomes.