Eukaryotic and Prokaryotic Phytochelatin Synthases Differ Less in Functional Terms Than Previously Thought: A Comparative Analysis of and sp. PCC 7407.

This paper reports functional studies on the enzyme phytochelatin synthase in the liverwort and the cyanobacterium sp. strain PCC 7407. In vitro activity assays in control samples (cadmium-untreated) showed that phytochelatin synthase was constitutively expressed in both organisms. In the presence of 100 µM cadmium, in both the liverwort and the cyanobacterium, the enzyme was promptly activated in vitro, and produced phytochelatins up to the oligomer PC. Likewise, exposure to 10-36 µM cadmium for 6-120 h induced in both organisms phytochelatin synthesis up to PC. Furthermore, the glutathione (GSH) levels in were constitutively low (compared with the average content in higher plants), but increased considerably under cadmium stress. Conversely, the GSH levels in sp. PCC 7407 were constitutively high, but were halved under metal treatments. At odds with former papers, our results demonstrate that, as in and other plants, the cyanobacterial phytochelatin synthase exposed to cadmium possesses manifest transpeptidasic activity, being able to synthesize phytochelatins with a degree of oligomerization higher than PC. Therefore, prokaryotic and eukaryotic phytochelatin synthases differ less in functional terms than previously thought.