Heavy metal and abiotic stress inducible metallothionein isoforms from Prosopis juliflora (SW) D.C. show differences in binding to heavy metals in vitro

Prosopis juliflora is a tree species that grows well in heavy metal laden industrial sites and accumulates heavy metals. To understand the possible contribution of metallothioneins (MTs) in heavy metal accumulation in P. juliflora, we isolated and compared the metal binding ability of three different types of MTs (PjMT1-3). Glutathione S-transferase fusions of PjMTs (GSTMT1-3) were purified from Escherichia coli cells grown in the presence of 0.3 mM cadmium, copper or zinc. Analysis of metal bound fusion proteins using atomic absorption spectrometry showed that PjMT1 bound higher levels of all three heavy metals as compared to PjMT2 and PjMT3. A comparative analysis of the genomic regions (including promoter for all three PjMTs) is also presented. All three PjMTs are induced by H₂O₂ and ABA applications. PjMT1 and PjMT2 are induced by copper and zinc respectively while PjMT3 is induced by copper, zinc and cadmium. Variation in induction of PjMTs in response to metal exposure and their differential binding to metals suggests that each MT has a specific role in P. juliflora. Of the three MTs analyzed, PjMT1 shows maximum heavy metal sequestration and is thus a potential candidate for use in heavy metal phytoremediation.