Endosymbiosis in trypanosomatids: the genomic cooperation between bacterium and host in the synthesis of essential amino acids is heavily influenced by multiple horizontal gene transfers

João Mp Alves,Flávia Da Silva,André G Costa-Martins,Maria Cristina M Motta,Ana Tereza R Vasconcelos,Gregory A Buck,Cecilia C Klein,Marta Mg Teixeira,Myrna G Serrano,Marie-France Sagot,Erney P Camargo

doi:10.1186/1471-2148-13-190

Abstract

BackgroundTrypanosomatids of the genera Angomonas and Strigomonas live in a mutualistic association characterized by extensive metabolic cooperation with obligate endosymbiotic Betaproteobacteria. However, the role played by the symbiont has been more guessed by indirect means than evidenced. Symbiont-harboring trypanosomatids, in contrast to their counterparts lacking symbionts, exhibit lower nutritional requirements and are autotrophic for essential amino acids. To evidence the symbiont’s contributions to this autotrophy, entire genomes of symbionts and trypanosomatids with and without symbionts were sequenced here.ResultsAnalyses of the essential amino acid pathways revealed that most biosynthetic routes are in the symbiont genome. By contrast, the host trypanosomatid genome contains fewer genes, about half of which originated from different bacterial groups, perhaps only one of which (ornithine cyclodeaminase, EC:4.3.1.12) derived from the symbiont. Nutritional, enzymatic, and genomic data were jointly analyzed to construct an integrated view of essential amino acid metabolism in symbiont-harboring trypanosomatids. This comprehensive analysis showed perfect concordance among all these data, and revealed that the symbiont contains genes for enzymes that complete essential biosynthetic routes for the host amino acid production, thus explaining the low requirement for these elements in symbiont-harboring trypanosomatids. Phylogenetic analyses show that the cooperation between symbionts and their hosts is complemented by multiple horizontal gene transfers, from bacterial lineages to trypanosomatids, that occurred several times in the course of their evolution. Transfers occur preferentially in parts of the pathways that are missing from other eukaryotes.ConclusionWe have herein uncovered the genetic and evolutionary bases of essential amino acid biosynthesis in several trypanosomatids with and without endosymbionts, explaining and complementing decades of experimental results. We uncovered the remarkable plasticity in essential amino acid biosynthesis pathway evolution in these protozoans, demonstrating heavy influence of horizontal gene transfer events, from Bacteria to trypanosomatid nuclei, in the evolution of these pathways.

Highlights

Trypanosomatids of the genera Angomonas and Strigomonas live in a mutualistic association characterized by extensive metabolic cooperation with obligate endosymbiotic Betaproteobacteria
In this work, the presence or absence of a given gene for a particular enzyme was verified in the genomes of endosymbionts, symbiont-harboring and regular trypanosomatids and compared with the available nutritional and enzymatic data on essential amino acid biosynthesis in insect trypanosomatids
Data on the genomes of leishmaniae and trypanosomes available in KEGG were used for comparison, but a wider sampling of genomes from more diverse groups of Trypanosomatidae and other, more distant Kinetoplastida will be necessary to enable more generalizing conclusions on the evolution of essential amino acid synthesis pathways in these organisms

Summary

Introduction

The Trypanosomatidae (Euglenozoa, Kinetoplastea) are well studied mainly because species of the genera Trypanosoma and Leishmania are pathogenic in humans and domestic animals [15] Despite their importance, these pathogens are a minority within the family, and most species are non-pathogenic commensals in the digestive tube of insects [16,17,18]. Trypanosomatids are nutritionally fastidious and require very rich and complex culture media, a very small group of these protozoa can be cultivated in very simple and defined media [19,20,21,22,23] This reduced group of insect trypanosomatids carries cytoplasmic endosymbionts and is known as symbiont-harboring trypanosomatids, to distinguish them from regular insect trypanosomatids naturally lacking symbionts. It has been shown that the symbiotic bacterium contains enzymes involved in host biosynthetic pathways, but in most cases the metabolic contribution of the endosymbiont has been inferred from nutritional data rather than genetically demonstrated [12,14]

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