Regulated expression at high copy number allows production of a growth-inhibitory oncogene product in Drosophila Schneider cells
The Drosophila metallothionein promoter (Mtn) was used to obtain efficient, regulated expression of foreign gene products inserted in high copy numbers into Drosophila melanogaster Schneider 2 cells. An expression unit comprised of a reporter gene [Escherichia coli galactokinase (galK)] fused to the Mtn promoter was stably introduced into Schneider 2 cells in up to several hundred copies per cell in a single transfection-selection event. This system contrasts dramatically with other eukaryotic systems that permit only a few copies of a gene to be stably inserted in a single transfection-selection event. The transfected Drosophila S2 cell lines expressed high levels of both galK mRNA and protein in response to metal induction. Most important, and in contrast to mammalian cells, expression remained fully regulated even at high copy number, with low basal expression maintained in the absence of inducer. This regulated system was used to obtain efficient expression in Drosophila cells of an otherwise lethal or growth-inhibitory gene product, the human H-ras oncogene. The ability to obtain regulated high-level expression of potentially lethal foreign proteins is unique to the Drosophila cell system.