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An HEK293S cell line resistant to ricin was prepared by mutagenesis by using ethyl methanesulfonate. It was shown to lack N-acetylglucosaminyltransferase I (GnTI) activity, and consequently unable to synthesize complex N-glycans. The tetracycline-inducible opsin expression system was assembled into this GnTI⁻ HEK293S cell line. Stable cell lines were isolated that gave tetracycline/sodium butyrate-inducible expression of the WT opsin gene at levels comparable with those observed in the parent tetracycline-inducible HEK293S cell line. Analysis of the N-glycan in rhodopsin expressed by the HEK293S GnTI⁻ stable cell line showed it to be Man₅GlcNAc₂. In a larger-scale expression experiment (1.1 liter) a WT opsin production level of 6 mg/liter was obtained. Further, the toxic constitutively active rhodopsin mutant, E113Q/E134Q/M257Y, previously shown to require inducible expression, has now been …