Horizontal gene transfer confers fermentative metabolism in the respiratory-deficient plant trypanosomatid <em>Phytomonas serpens</em>

Among trypanosomatids, the genus Phytomonas is the only one specifically adapted to infect plants. These hosts provide a particular habitat with a plentiful supply of carbohydrates. Phytomonas sp. lacks a cytochrome-mediated respiratory chain and Krebs cycle, and ATP production relies predominantly on glycolysis. We have characterised the complete gene encoding a putative pyruvate/indolepyruvate decarboxylase (PDC/IPDC) (548 amino acids) of P. serpens, that displays high amino acid sequence similarity with phytobacteria and Leishmania enzymes. No orthologous PDC/IPDC genes were found in Trypanosoma cruzi or T. brucei. Conservation of the PDC/IPDC gene sequence was verified in 14 Phytomonas isolates. A phylogenetic analysis shows that Phytomonas protein is robustly monophyletic with Leishmania spp. and C. fasciculata enzymes. In the trees this clade appears as a sister group of indolepyruvate decarboxylases of γ-proteobacteria. This supports the proposition that a horizontal gene transfer event from a donor phytobacteria to a recipient ancestral trypanosome has occurred prior to the separation between Phytomonas, Leishmania and Crithidia. We have measured the PDC activity in P. serpens cell extracts. The enzyme has a Km value for pyruvate of 1.4mM. The acquisition of a PDC, a key enzyme in alcoholic fermentation, explains earlier observations that ethanol is one of the major end-products of glucose catabolism under aerobic and anaerobic conditions. This represents an alternative and necessary route to reoxidise part of the NADH produced in the highly demanding glycolytic pathway and highlights the importance of this type of event in metabolic adaptation.

在锥虫类中，植滴虫属是唯一专门适应感染植物的属。这些宿主提供了一个富含碳水化合物的特殊生境。植滴虫缺乏细胞色素介导的呼吸链和三羧酸循环，ATP的产生主要依赖糖酵解。我们已经对蛇形植滴虫中一个假定的丙酮酸/吲哚丙酮酸脱羧酶（PDC/IPDC）（548个氨基酸）的完整基因进行了特征分析，它与植物细菌和利什曼原虫的酶具有高度的氨基酸序列相似性。在克氏锥虫或布氏锥虫中未发现直系同源的PDC/IPDC基因。在14个植滴虫分离株中验证了PDC/IPDC基因序列的保守性。系统发育分析表明，植滴虫的蛋白质与利什曼原虫属和簇虫属的酶具有很强的单系性。在系统发育树中，这个分支作为γ - 变形菌吲哚丙酮酸脱羧酶的姊妹群出现。这支持了这样一种观点，即从供体植物细菌到受体祖先锥虫的水平基因转移事件发生在植滴虫、利什曼原虫和短膜虫分离之前。我们测量了蛇形植滴虫细胞提取物中的PDC活性。该酶对丙酮酸的Km值为1.4 mM。获得PDC（酒精发酵中的一种关键酶）解释了早期的观察结果，即乙醇是有氧和无氧条件下葡萄糖分解代谢的主要终产物之一。这代表了一种重新氧化在高需求糖酵解途径中产生的部分NADH的替代且必要的途径，并凸显了这种类型的事件在代谢适应中的重要性。