Characterisation of novel functionality within the Blastocystis tryptophanase gene

In recent years, the human gut microbiome has been recognised to play a pivotal role in the health of the host. Intestinal homeostasis relies on this intricate and complex relationship between the gut microbiota and the human host. While much effort and attention has been placed on the characterization of the organisms that inhabit the gut microbiome, the complex molecular cross-talk between the microbiota could also exert an effect on gastrointestinal conditions. Blastocystis is a single-cell eukaryotic parasite of emerging interest, as its beneficial or pathogenic role in the microbiota has been a subject of contention even to-date. In this study, we assessed the function of the Blastocystis tryptophanase gene (BhTnaA), which was acquired by horizontal gene transfer and likely to be of bacterial origin within Blastocystis. Bioinformatic analysis and phylogenetic reconstruction revealed distinct divergence of BhTnaA versus known bacterial homologs. Despite sharing high homology with the E. coli tryptophanase gene, we show that Blastocystis does not readily convert tryptophan into indole. Instead, BhTnaA preferentially catalyzes the conversion of indole to tryptophan. We also show a direct link between E. coli and Blastocystis tryptophan metabolism: In the presence of E. coli, Blastocystis ST7 is less able to metabolise indole to tryptophan. This study examines the potential for functional variation in horizontally-acquired genes relative to their canonical counterparts, and identifies Blastocystis as a possible producer of tryptophan within the gut. Author summaryBlastocystis is a genus of microscopic parasites able to infect the human colon. Whether it plays a role in disease has been a topic of debate in the scientific community since its discovery. Some evidence links infection with the parasite to an increase in biodiversity in gut bacteria, which is associated improvements in health. Other evidence links Blastocystis to symptoms of the common gut disease irritable bowel syndrome, or IBS. This study shows that Blastocystis interacts with the amino acid tryptophan in an unexpected manner, which may shed light how it might cause disease. We show that it uses an enzyme known as tryptophanase in reverse-in other organisms, tryptophanase converts tryptophan into another molecule known as indole, while Blastocystis converts indole to tryptophan. Cells in the gut called enterochromaffin cells convert tryptophan into the neurotransmitter serotonin. This molecule can influence the behaviour of the gut, and has been linked to symptoms of IBS. We believe that, under some circumstances, an infection with Blastocystis may cause an increase in tryptophan in the gut, which is then converted to serotonin. The excess serotonin would then cause symptoms of the disease.