Redox modulation of the expression of bacterial genes encoding cysteine-rich proteins in plant protoplasts

Activity of neomycin phosphotransferase II (NPTII; gene, neo; five cysteines) in tobacco protoplasts transfected with fusions of the octopine TR2' or cauliflower mosaic virus 35S promoter and the neo gene, with or without a signal peptide, increased up to 8-fold in response to externally added dithiothreitol at concentrations that did not affect protoplast viability (up to 2.5 mM). Activity of phosphinothricin acetyltransferase (PAT; gene, bar; one cysteine) expressed under control of the TR1' or 35S promoter was not similarly affected, thus excluding a redox modulation of transcription as the mechanism of NPTII activation by dithiothreitol. Western-blot analyses showed an increase in the amount of protein in response to dithiothreitol, whereas neither the steady-state level of NPTII mRNA nor the specific activity of the purified enzyme was affected. The same type of modulation was observed for transiently expressed beta-glucuronidase (nine cysteines) produced from a fusion with the 35S promoter, with or without a signal peptide. Limitation of cotranslational and/or early posttranslational steps by excessively oxidizing sulfhydryl/disulfide redox potentials is postulated to explain the low net accumulation of cysteine-rich proteins of bacterial origin (i.e., NPTII and beta-glucuronidase) when expressed in plant protoplasts, and the marked increase in such proteins in response to externally added dithiothreitol.