Characterization of a cDNA encoding metallothionein 3 from cotton (Gossypium hirsutum L.)

A cDNA encoding metallothionein (MT) was isolated from a library constructed with poly A(+) RNA purified from 48 h etiolated cotton (Gossypium hirsutum L.) cotyledons. This cDNA encodes a deduced protein with 63 residues and a molecular weight of 6.3 kDa. The protein has 10 cysteines of which 4 are within the CXXCXCXXXXXC amino-terminus motif and six are within the CXCXXXCXCXXCXC carboxyl-terminus motif characteristic of the type III MT (MT3). The cotton MT3 protein sequence is 76.2, 69.8, 66.7, 60.3 and 33.5% identical to MT3 from Carica papaya, Rubus idaeus, Ribes nigrum, Citrus unshiu, and Gossypium hirsutum type I MT, respectively. A fusion protein was constructed by producing PCR primers for the 5' and 3' ends of the cotton MT3 cDNA and ligating the PCR product inframe at the 3' end of a bacterial glutathione S-transferase (GST) gene in the pGEX3 vector. The 5' PCR primer incorporated a segment of the cotton MT3 noncoding region, resulting in an addition of 9 residues to the MT3 (after Factor Xa digestion site) which increased the size of the expressed protein to 72 residues and 7.6 kDa. Expression of the 7.6 kDa protein in bacteria was confirmed by SDS-PAGE. Induction and accumulation of the GST-MT3 protein began inhibiting bacterial growth after 1 h. Addition of Cu (1 muM to 1 mM), 1 mM cysteine, or 1 mM cystine to the media did not rescue growth. Additionally, this protein was evaluated for its ability to bind Cd, Cu, Ni and Zn in the bacterial expression system. We found that cotton MT3 preferentially binds Cu.