High frequency of stable 5-methyl-DL-tryptophan resistance in Brassica nigra cell suspension cultures

Suspension-cultured cells of Brassica nigra resistant to growth inhibition by 5-methyl-DL-tryptophan were isolated at a high frequency estimated to be in the range of 5×10^–4 to 7×10^–3. A resistance of 20-fold was stable in the absence of selective pressure. A subline with a stable resistance of 115-fold was derived by 3 months growth in the presence of high concentrations of the analogue. The target enzyme, anthranilate synthetase, appeared normal in the variant but levels of alanine, glutamine and related amino acids were elevated. That this change may confer resistance to 5MT was demonstrated by supplementing the growth medium of sensitive cells with alanine, glutamine and glutamate.