Expression of mitochondrial malate dehydrogenase in Escherichia coli improves phosphate solubilization

Penicillium oxalicum C2 was isolated from the rhizosphere of Mexican stonecrop based on its high level of tricalcium phosphate solubilizing activity. A full-length gene encoding mitochondrial malate dehydrogenase (mMDH) was cloned from P. oxalicum C2 by RT-PCR and RACE techniques. The cDNA was 1,284 bp in length, with a complete open reading frame of 1,023 bp encoding a protein of 340 amino acids with a predicted molecular mass of 35.7 kDa. The predicted amino acid sequence shared high identity with mMDHs from other organisms. Escherichia coli strain BL21(DE3) transformed with the P. oxalicum C2 mMDH gene showed an increase in MDH activity of about 5-fold compared to non-transformed cell as measured in cell extracts. It also secreted more malate, lactate, acetate, citrate, oxalate and produced higher clearing halos when grown in plates with tricalcium phosphate as the sole source of phosphate. These results showed that expression of the P. oxalicum C2 mMDH gene in E. coli could enhance organic acid secretion and improve the phosphate solubilizing ability of the cell.