EXPRESSION OF THE TYPE-2 METALLOTHIONEIN-LIKE GENE MT2 FROM ARABIDOPSIS-THALIANA IN ZN2-METALLOTHIONEIN-DEFICIENT SYNECHOCOCCUS PCC-7942 - PUTATIVE ROLE FOR MT2 IN ZN2+ METABOLISM()

Zn2+ proteins pervade metabolism and are essential for gene expression . However, no proteins have been ascribed the central roles of Zn2+ do nation to, or removal from, metalloproteins, or Zn2+ storage in vegeta tive plant tissue. In animals, such functions have been proposed for m etallothioneins. Plants contain multiple metallothionein-like genes bu t their predicted products, which differ significantly from animal met allothioneins, remain to be isolated from vegetative tissue and their roles are uncertain. The type 2 metallothionein-like gene from Arabido psis, MT2, was expressed under the control of Zn2+ -responsive element s derived from the cyanobacterial metallothionein divergon, smt. Zn2-dependent expression of MT2 transcripts in Synechococcus PCC 7942 was confirmed by northern analysis. The Ambidopsis MT2 gene partly comple mented Zn2+ hypersensitivity in mutants of Synechococcus PCC 7942 whic h are functionally deficient in an endogenous Zn2+ -metallothionein ge ne, smtA. MT2 was also expressed as a recombinant fusion protein in Es cherichia coli, purified and shown to bind Zn2+ in vitro. The mean pH of half displacement of Zn2+ from MT2 was estimated to be 5.05. This s uggests that MT2 has a greater affinity for Zn2+ than phytochelatins. The results presented here reveal that MT2 is capable of binding Zn2in vitro, conferring tolerance to elevated [Zn2+] in vivo within cyano bacteria and is likely to compete with other polypeptides for cellular Zn2+ in planta.