Abstract: The cyanobacterium Synechocystis sp. PCC 6803 was cultivated for 5 d in sterile liquid medium BG-11 at room temperature under fluorescent light, aerated by stirring under sterile condition. ZnCl2 was then added into the medium at final concentration of 50 μmol/L to induce the expression of the metallothionein-like (MT-like) compound. Five d later, the ZnCl2 concentration was increased to 100 μmol/L. After 2 d, cells were collected by centrifugation, then disrupted by ultrasonic wave. After centrifugation, the supernatant was treated at 72 ℃ for 3 min to denature the unstable proteins. Again after centrifugation, the supernatant was applied to Sephacryl S-l00 molecular sieve column, then DEAE-Sepharose F. F. ion exchange column and Sephadex G-25 column for desalination. The Zn-binding fraction was frozen dried. In 5 L cultures, 7.6 g wet cyanobacterial cells were obtained. Each time the lysate of 1.52 g of them was processed, finally 1.5 mg purified MT-like compound was obtained, which was 0.1% of the total weight of wet cyanobacterial cells. Isoelectric focusing (IEF) showed that its pI was pH 4.5. Mass spectrometry and amino acid composition analysis indicated that the molecular weight of the protein was 6.986 kD. Amino acid composition analysis showed that the content of hydrophobic amino acid residues was 36%, but the Cys content was not very high (only 5% ). Ultraviolet absorption spectra showed that Zn-MT-like binding complex also had a high absorbance at 220 nm. CD spectroscopy proved that the secondary structure of this protein was a random coil, and that there were no α-helix and β-sheet in the structure. FT-IR spectroscopy of cyanobacterial MT-like compound was similar to that of the typical MT of mammal.

Key words: Synechocystis sp. PCC 6803, Metallothionein-like compound, Isolation and purification, Characteriation, Conformation in solution