I'm planning on generating conditional knockout cell lines of apparently essential genes to test the functionality of some fusion proteins in bloodstream form T. brucei. The idea is to knockout the endogenous copies while ectopically expressing a rescue version - with or without the fusion (using pLEW100v5 construct) - under the control of Tetracycline in the "Single Marker" cell line, or one transfected with the SMOx construct from Steve Kelly.

Unfortunately, the expression levels of my genes of interest are highly critical, so that over-expression is lethal and even generating a single allele knockout is very difficult. To overcome this, I was planning on repeating the knockout transfections while inducing expression of the 'rescue' copy at different levels by varying the tetracycline concentration - hopefully I'll hit upon a concentration that results in the right amount of expression to complement the knockout of each individual allele.

My question is (Finally!): can anyone recommend a sensible range of Tetracycline concentrations that will allow me to vary the amount of expression? Does anyone know whether expression level is linearly related to Tetracycline concentration?

Any advice would be greatly appreciated!

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