Expression of Mammalian Metallothionein Suppresses Glucosinolate Synthesis in Brassica campestris

Transfection of Brassica campestris leaves with cauliflower mosaic virus (CaMV) harboring a mammalian metallothionein (MT) cDNA at the ORFII position lowered the glucosinolate (GS) concentration to approximately one-half the level in leaves infected with wild-type CaMV. This suppression was independent of the plant's sulfate status, suggesting that the pathways for protein (MT) and GS biosynthesis were competing for S on an equal basis. The expression of MT may have lowered the endogenous levels of Cys, 3'-phosphoadenosine-5'-phosphosulfate, or Met, all of which are required for GS synthesis in B. campestris. These results indicate that the introduction of structural genes coding for high levels of specific amino acids can be used to alter the production of nonproteinaceous molecules within plants.