Horizontal Gene Transfer of Phytochelatin Synthases from Bacteria to Extremophilic Green Algae

Transcriptomic sequencing together with bioinformatic analyses and an automated annotation process led us to identify novel phytochelatin synthase (PCS) genes from two extremophilic green algae (Chlamydomonas acidophila and Dunaliella acidophila). These genes are of intermediate length compared to known PCS genes from eukaryotes and PCS-like genes from prokaryotes. A detailed phylogenetic analysis gives new insight into the complicated evolutionary history of PCS genes and provides evidence for multiple horizontal gene transfer events from bacteria to eukaryotes within the gene family. A separate subgroup containing PCS-like genes within the PCS gene family is not supported since the PCS genes are monophyletic only when the PCS-like genes are included. The presence and functionality of the novel genes in the organisms were verified by genomic sequencing and qRT-PCR. Furthermore, the novel PCS gene in Chlamydomonas acidophila showed very strong induction by cadmium. Cloning and expression of the gene in Escherichia coli clearly improves its cadmium resistance. The gene in Dunaliella was not induced, most likely due to gene duplication.