Comparative Transcriptome Analysis of a Toxin-Producing Dinoflagellate Alexandrium catenella and Its Non-Toxic Mutant

Abstract

The dinoflagellates and cyanobacteria are two major kingdoms of life producing paralytic shellfish toxins (PSTs), a large group of neurotoxic alkaloids causing paralytic shellfish poisonings around the world. In contrast to the well elucidated PST biosynthetic genes in cyanobacteria, little is known about the dinoflagellates. This study compared transcriptome profiles of a toxin-producing dinoflagellate, Alexandrium catenella (ACHK-T), and its non-toxic mutant form (ACHK-NT) using RNA-seq. All clean reads were assembled de novo into a total of 113,674 unigenes, and 66,812 unigenes were annotated in the known databases. Out of them, 35 genes were found to express differentially between the two strains. The up-regulated genes in ACHK-NT were involved in photosynthesis, carbon fixation and amino acid metabolism processes, indicating that more carbon and energy were utilized for cell growth. Among the down-regulated genes, expression of a unigene assigned to the long isoform of sxtA, the initiator of toxin biosynthesis in cyanobacteria, was significantly depressed, suggesting that this long transcript of sxtA might be directly involved in toxin biosynthesis and its depression resulted in the loss of the ability to synthesize PSTs in ACHK-NT. In addition, 101 putative homologs of 12 cyanobacterial sxt genes were identified, and the sxtO and sxtZ genes were identified in dinoflagellates for the first time. The findings of this study should shed light on the biosynthesis of PSTs in the dinoflagellates.

Current Knowledge and Recent Advances in Marine Dinoflagellate Transcriptomic Research

“…The A. catenella non-toxic mutant showed lower expression of the long isoform of the dinoflagellate sxtA gene, which is responsible for the initiation of toxin biosynthesis. However, the expression of the short isoform of the dinoflagellate sxtA gene showed no significant different between the toxic and non-toxic mutant strain of A. catenella [67]. Some toxin-related genes also have been reported to be widely present in non-toxic dinoflagellates [70].…”

“…The dinoflagellate sxtA transcript differs from its cyanobacterial counterpart by the presence of signal peptides, SL sequences and polyA-tails and also in its GC content [69]. Comparative transcriptome analysis of Alexandrium catenella with its non-toxic mutant showed that 35 genes were found to be differentially expressed between these two strains [67]. The authors of this study also speculated the A. catenella non-toxic mutant has more active carbon fixation and a higher ATP requirement due to higher expression of genes involved in cellular metabolism such as fructose biphosphate aldolase.…”

“…Comparative transcriptome analysis of toxic versus non-toxic dinoflagellates is one of the strategies used by researchers to elucidate the molecular mechanism of PSP toxin biosynthesis [67,68].…”

Gene duplication, loss and selection in the evolution of saxitoxin biosynthesis in alveolates

“…Only limited information is available regarding the expression and function of sxtA copies (Perini et al, 2014;Zhang et al, 2014). It appears that sxtA may not be strongly regulated at the transcriptional level in A. minutum, as toxin quantities were not well correlated with transcript abundance (Perini et al, 2014).…”

“…It appears that sxtA may not be strongly regulated at the transcriptional level in A. minutum, as toxin quantities were not well correlated with transcript abundance (Perini et al, 2014). Nevertheless, sxtA was found to be significantly down regulated in a non-toxic mutant strain of A. pacificum (Zhang et al, 2014). There are several environmental factors that appear to influence the amount of STX found in the cell (reviewed in Murray et al, 2015).…”