EXPRESSION AND RNA SPLICING OF THE MAIZE GLUTATHIONE-S-TRANSFERASE BRONZE2 GENE IS REGULATED BY CADMIUM AND OTHER STRESSES

The Bronze2 (Bz2) gene in maize (Zea mays) encodes a glutathione S-tra nsferase that performs the last genetically defined step in anthocyani n biosynthesis-tagging anthocyanin precursors with glutathione, allowi ng for recognition and entry of anthocyanins into the vacuole. Here we show that Bz2 gene expression is highly induced by heavy metals such as cadmium. Treatment of maize seedlings with cadmium results in a 20- fold increase in Bz2 message accumulation and a 50-fold increase in th e presence of the unspliced, intron-containing transcript. The increas e in message levels during cadmium stress appears to result, at least in part, from activation of an alternative mRNA start site approximate ly 200 nucleotides upstream of the normal start site; this site is not used in unstressed or heat-stressed tissues. The effect of cadmium on the RNA splicing of Bz2 seems to be specific: splicing of other intro n-containing maize genes, including a maize actin gene under the contr ol of the cadmium-inducible Bz2 promoter, is unaffected by cadmium str ess. Conversely, Bz2 intron splicing is not affected by other stress c onditions that induce Bz2 gene expression, such as abscisic acid, auxi n, or cold stress. Surprisingly, the increase in Bz2 mRNA during cadmi um stress does not result in an increase in Bz2 glutathione S-transfer ase activity. We propose that an alternative protein may be encoded by Bz2 that has a role during responses to heavy metals.