I'm trying to make a stable transgenic line of a rather delicate eukaryote, so I want to activate transgene expression inducibly, but that means I need to express some kind of intermediary protein like rtTA constitutively. I know promoters activity varies significantly between species, but does anybody have any suggestions for a mild promoter I can use to express rtTA that won't put undue stress on my organism? Or another tangential question, what is the least toxic transactivator? Thanks.