GENES WITH SIMILARITY TO METALLOTHIONEIN GENES AND COPPER, ZINC LIGANDS IN PISUM-SATIVUM L

The PsMT gene family of pea (Pisum sativum L.) encodes predicted proteins with sequence similarity to metallothioneins. However, PsMT proteins have not yet been characterised in planta and their functions remain obscure. PsMT transcripts were identified in the cortex tissue of pea roots using tissue squash-blotting techniques. Transcripts were not detected on northern blots of RNA isolated from the embryonic radicle, but PsMT transcript abundance in roots increased with age of germinating seedlings. The PsMT(A) gene was expressed in E. coli as a carboxyterminal extension of glutathione-S-transferase (GST). Fusion protein purified from crude cell lysates (500 mL cultures) bound an estimated amount of 5.99, 6.27 and 7.07 moles of Zn, Cu and Cd respectively per mole protein, compared to equivalent estimates of 0.37, 0.63 and 0.26 moles for GST alone. Similar estimates for Fe-binding were 0.28 moles for GST-PsMT(A) fusion protein and 0.1 moles for GST alone. In summary, these data: 1, show that PsMT transcripts are abundant in roots of pea plants that have not been exposed to supra-optimal concentrations of trace metals and hence appear to be constitutively expressed and 2, indicate that PsMT(A) protein can bind certain trace metal ions. We have also identified and partially purified a Zn ligand (Zn-A) and two Cu ligands (Cu-A, Cu-B) from pea roots which have not been exposed to supra-optimal conditions of trace metal ions and are therefore defined as 'constitutive'. Whether or not these ligands include the products of PsMT genes remains to be established.