RESUMO
A modified tetracycline-responsive expression system (TRES) for use in insect cells was developed. The TRES contains two components: one encodes a tetracycline-controllable transactivator (tTA) and the other contains a tet operator DNA sequence to drive the luciferase gene. Our results show that the human cytomegalovirus (CMV) promoter, an essential part for strong tTA expression in mammalian system, was not functional in insect cells. Thus further modifications were required. Functional tTA was efficiently expressed in Sf9, Sf21, and TN368 cells by the p10 promoter of Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) in plasmid form with virus co-infection. An increase of up to 258-fold of luciferase activity was detected in these cells when both components in modified TRES were co-transfected. In order to further simplify the experiment, tTA, which is driven by the p10 promoter, was inserted into AcMNPV. Luciferase activity was also strongly stimulated by the infection of this tTA expression-recombinant virus with the transfection of a plasmid containing the second TRES component expressing luciferase. The luciferase expressions in these systems, either in plasmids or the tTA gene in virus and luciferase in plasmid, were significantly suppressed by tetracycline. The time course kinetics of tetracycline action to the TRES were further studied. Within a time span of 50 h, the luciferase activities could be fully suppressed or activated, respectively, corresponding to the addition or removal of tetracycline. These experiments have established a well-regulated gene expression system for further broad applications of molecular biological studies in insect cells.