The G‐box (CACGTG) and H‐box (CCTACC) cis elements function in the activation of phenylpropanoid biosynthetic genes involved in the elaboration of lignin precursors, phytoalexins and the secondary signal salicylic acid as early responses to pathogen attack. We have isolated a soybean cDNA encoding a novel bZIP protein, G/HBF‐1, which binds to both the G‐box and adjacent H‐box in the proximal region of the chalcone synthase chs15 promoter. While G/HBF‐1 transcript and protein levels do not increase during the induction of phenylpropanoid biosynthetic genes, G/HBF‐1 is phosphorylated rapidly in elicited soybean cells, almost exclusively on serine residues. Using recombinant G/HBF‐1 as a substrate, we identified a cytosolic protein‐serine kinase that is rapidly and transiently stimulated in cells elicited with either glutathione or an avirulent strain of the soybean pathogen Pseudomonas syringae pv. glycinea. Phosphorylation of G/HBF‐1 in vitro enhances binding to the chs15 promoter and we conclude that stimulation of G/HBF‐1 kinase activity and G/HBF‐1 phosphorylation are terminal events in a signal pathway for activation of early transcription‐dependent plant defense responses.

The PsMTA gene from pea (Pisum sativum) shares similarity with metallothionein (MT) genes and related sequences have also been isolated from a number of other higher-plant species. The proteins encoded by these genes have not yet been purified from plants and their functions remain unclear although, by analogy to MT, roles in the metabolism and detoxification of metal ions have been proposed. By contrast, correlation between transcript abundance and Fe availability has led to an alternative proposal that these genes are involved in mechanisms of Fe efficiency. Phenotypic effects of constitutive PsMTA expression were examined in Escherichia coli and Arabidopsis thaliana. Copper accumulation by E. coli cells expressing recombinant PsMTA protein was approximately 8-fold greater than in control cells. No significant effects on the accumulation of Zn or Cd were detected. In segregating A. thaliana progeny, derived from a transgenic F1 parent containing the PsMTA gene under the control of a CaMV 35S promoter, 75% of individuals accumulated more Cu (several-fold in some plants) than untransformed, control plants. These data suggest that PsMTA protein binds Cu in planta and that uncoupled (constitutive) expression of the PsMTA gene causes enhanced Cu accumulation. Roots of P. sativum plants grown under conditions of low Fe availability showed elevated activity of root surface Fe(III) reductase and accumulated more Cu than roots of plants grown in an Fe-supplemented solution. Changes in the expression of MT-like genes, coincident with changes in Fe availability, are consistent with a role in Cu homoeostasis.

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The recently isolated Synechococcus gene satr/l encodes the only character&d prokaryotic protein designated to k a metallothionein (MT). To examine the melal-binding properties of its product the snlrA gene was expressed in Escitericiriu coii as a carboxyterminal extension of glutathionc-S-transfcrase. The pH of half dissociation of Zn, Cd and Cu ions from the expressed protein was determined to be 4.10. 3.50, 2.35. respectively. indicating a high affinity for these ions (in particular for Zn in comparison to mammalian MT). E. co/i expressing this gene showed enhanced (ca.3-fold) accumulation of Zn.

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