Phytochelatin synthase in cyanobacteria and eukaryotic microalgae.

Phytochelatins (PCs) are low molecular weight cysteine-rich metal binding peptides that are not genetically encoded but enzymatically synthesized from glutathione by Phytochelatin Synthase (PCS) (Grill et al. 1989). This enzyme is constitutively expressed, even in the absence of metal contamination, is phylogenetically distributed in a wide range of organisms, and its active site shares similarity with the papain type cysteine proteases catalytic domain (Rea et al. 2004). Altogether, these observations suggest a wide range of functions for PCS (Cobbet and Goldsbrough. 2002). These proteins have been extensively studied in land plants (Filiz et al. 2019), but till now not fully analyzed in algae and cyanobacteria despite these organisms are the first to cope with heavy metal stress in aquatic environments and their potentiality for phytoremediation. To fill this gap, we conducted a comparative analysis of the features of PCS proteins of different cyanobacterial and algal taxa, with particular focus on their phylogenetic linkage (Ferrari et al. 2024). Our results indicate that the examined sequences fall into two main, already known, groups of PCS-like proteins and that, contrary to previous assumptions, they do not split between prokaryotic and eukaryotic sequences, but rather into group of sequences distinguishable by the alternative presence of asparagine (“N group”) and glutamic/aspartic acid (“E/D group”) residues in proximity of the catalytic cysteine. In both branches we found both prokaryotic and eukaryotic sequences. Diatoms, red algae, cyanobacteria and a group of extremophilic Chlamydomonadales possess both “N” and “E/D” isoforms. The analysis of protein parameters of some model organisms, chosen as representative of different taxa, indicated that cyanobacterial proteins, regardless of to the group they belong, have an elevated aliphatic index, suggesting a greater thermostability than those of eukaryotic proteins. The greater stability predicted for “N” group proteins could explain their presence in marine eukaryotic or extremophilic algae, which also possess the “E” PCS enzyme, suggesting that “N” isoforms are important in responding to particular environments adaptations. The existence of two kind of the enzyme with distinct features and putative differences in posttranslational regulation suggests their different use in relation to cell/environmental requirements and/or to different cell functions rather than exclusively attributable to phylogenetic divergence and lead us to rethink what in previous works was indicated as the difference between cyanobacterial and eukaryotic PCSs.