Plastids, organelles that evolved from cyanobacteria via endosymbiosis in eukaryotes, provide carbohydrates for the formation of biomass and for mitochondrial energy production to the cell. They generate their own energy in the form of the nucleotide adenosine triphosphate (ATP). However, plastids of non-photosynthetic tissues, or during the dark, depend on external supply of ATP. A dedicated antiporter that exchanges ATP against adenosine diphosphate (ADP) plus inorganic phosphate (Pi) takes over this function in most photosynthetic eukaryotes. Additional forms of such nucleotide transporters (NTTs), with deviating activities, are found in intracellular bacteria, and, surprisingly, also in diatoms, a group of algae that acquired their plastids from other eukaryotes via one (or even several) additional endosymbioses compared to algae with primary plastids and higher plants. In this review, we summarize what is known about the nucleotide synthesis and transport pathways in diatom cells, and discuss the evolutionary implications of the presence of the additional NTTs in diatoms, as well as their applications in biotechnology.

Institute of Parasitology Biology Centre Czech Academy of Sciences Branišovská 1160 31 370 05 České Budějovice Czech Republic

Pflanzenphysiologie Technische Universität Kaiserslautern 67663 Kaiserslautern Germany

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Field C.B. Primary Production of the Biosphere: Integrating Terrestrial and Oceanic Components. Science. 1998;281:237–240. doi: 10.1126/science.281.5374.237. PubMed DOI

Armbrust E.V. The life of diatoms in the world’s oceans. Nature. 2009;459:185–192. doi: 10.1038/nature08057. PubMed DOI

Falkowski P.G., Katz M.E., Knoll A.H., Quigg A., Raven J.A., Schofield O., Taylor F.J. The evolution of modern eukaryotic phytoplankton. Science. 2004;305:354–360. doi: 10.1126/science.1095964. PubMed DOI

Archibald J.M. Genomic perspectives on the birth and spread of plastids. Proc. Natl. Acad. Sci. USA. 2015 doi: 10.1073/pnas.1421374112. PubMed DOI PMC

Archibald J.M. Endosymbiosis and Eukaryotic Cell Evolution. Curr. Biol. CB. 2015;25:R911–R921. doi: 10.1016/j.cub.2015.07.055. PubMed DOI

Brodie J., Ball S.G., Bouget F.-Y., Chan C.X., De Clerck O., Cock J.M., Gachon C., Grossman A.R., Mock T., Raven J.A., et al. Biotic interactions as drivers of algal origin and evolution. New Phytol. 2017;216:670–681. doi: 10.1111/nph.14760. PubMed DOI

Oborník M. Endosymbiotic Evolution of Algae, Secondary Heterotrophy and Parasitism. Biomolecules. 2019;9:266. doi: 10.3390/biom9070266. PubMed DOI PMC

Baurain D., Brinkmann H., Petersen J., Rodriguez-Ezpeleta N., Stechmann A., Demoulin V., Roger A.J., Burger G., Lang B.F., Philippe H. Phylogenomic evidence for separate acquisition of plastids in cryptophytes, haptophytes, and stramenopiles. Mol. Biol. Evol. 2010;27:1698–1709. doi: 10.1093/molbev/msq059. PubMed DOI

Dorrell R.G., Gile G., McCallum G., Méheust R., Bapteste E.P., Klinger C.M., Brillet-Guéguen L., Freeman K.D., Richter D.J., Bowler C. Chimeric origins of ochrophytes and haptophytes revealed through an ancient plastid proteome. eLife. 2017;6:e23717. doi: 10.7554/eLife.23717. PubMed DOI PMC

Nowack E.C.M., Weber A.P.M. Genomics-Informed Insights into Endosymbiotic Organelle Evolution in Photosynthetic Eukaryotes. Ann. Rev. Plant Biol. 2018;69:51–84. doi: 10.1146/annurev-arplant-042817-040209. PubMed DOI

Gould S.B., Waller R.F., McFadden G.I. Plastid evolution. Ann. Rev. Plant Biol. 2008;59:491–517. doi: 10.1146/annurev.arplant.59.032607.092915. PubMed DOI

Kroth P.G. Protein transport into secondary plastids and the evolution of primary and secondary plastids. Int. Rev. Cytol. 2002;221:191–255. doi: 10.1016/S0074-7696(02)21013-X. PubMed DOI

Flori S., Jouneau P.-H., Finazzi G., Maréchal E., Falconet D. Ultrastructure of the Periplastidial Compartment of the Diatom Phaeodactylum tricornutum. Protist. 2016;167:254–267. doi: 10.1016/j.protis.2016.04.001. PubMed DOI

Gruber A., Weber T., Rio Bartulos C., Vugrinec S., Kroth P.G. Intracellular distribution of the reductive and oxidative pentose phosphate pathways in two diatoms. J. Basic Microbiol. 2009;49:58–72. doi: 10.1002/jobm.200800339. PubMed DOI

Moog D., Stork S., Zauner S., Maier U.-G. In Silico and In Vivo Investigations of Proteins of a Minimized Eukaryotic Cytoplasm. Genome Biol. Evol. 2011;3:375–382. doi: 10.1093/gbe/evr031. PubMed DOI PMC

Weber T., Gruber A., Kroth P.G. The Presence and Localization of Thioredoxins in Diatoms, Unicellular Algae of Secondary Endosymbiotic Origin. Mol. Plant. 2009;2:468–477. doi: 10.1093/mp/ssp010. PubMed DOI

Ball S.G., Morell M.K. From Bacterial Glycogen to Starch: Understanding the Biogenesis of the Plant Starch Granule. Ann. Rev. Plant Biol. 2003;54:207–233. doi: 10.1146/annurev.arplant.54.031902.134927. PubMed DOI

Deschamps P., Haferkamp I., d’Hulst C., Neuhaus H.E., Ball S.G. The relocation of starch metabolism to chloroplasts: When, why and how. Trends Plant Sci. 2008;13:574–582. doi: 10.1016/j.tplants.2008.08.009. PubMed DOI

Ball S., Colleoni C., Cenci U., Raj J.N., Tirtiaux C. The evolution of glycogen and starch metabolism in eukaryotes gives molecular clues to understand the establishment of plastid endosymbiosis. J. Exp. Bot. 2011;62:1775–1801. doi: 10.1093/jxb/erq411. PubMed DOI

Haferkamp I., Fernie A.R., Neuhaus H.E. Adenine nucleotide transport in plants: Much more than a mitochondrial issue. Trends Plant Sci. 2011;16:507–515. doi: 10.1016/j.tplants.2011.04.001. PubMed DOI

Linka N., Hurka H., Lang B.F., Burger G., Winkler H.H., Stamme C., Urbany C., Seil I., Kusch J., Neuhaus H.E. Phylogenetic relationships of non-mitochondrial nucleotide transport proteins in bacteria and eukaryotes. Gene. 2003;306:27–35. doi: 10.1016/S0378-1119(03)00429-3. PubMed DOI

Weber A.P., Linka N. Connecting the plastid: Transporters of the plastid envelope and their role in linking plastidial with cytosolic metabolism. Ann. Rev. Plant Biol. 2011;62:53–77. doi: 10.1146/annurev-arplant-042110-103903. PubMed DOI

Trentmann O., Jung B., Neuhaus H.E., Haferkamp I. Nonmitochondrial ATP/ADP transporters accept phosphate as third substrate. J. Biol. Chem. 2008;283:36486–36493. doi: 10.1074/jbc.M806903200. PubMed DOI PMC

Huang W., Haferkamp I., Lepetit B., Molchanova M., Hou S., Jeblick W., Río Bártulos C., Kroth P.G. Reduced vacuolar β-1,3-glucan synthesis affects carbohydrate metabolism as well as plastid homeostasis and structure in Phaeodactylum tricornutum. Proc. Natl. Acad. Sci. USA. 2018;115:4791–4796. doi: 10.1073/pnas.1719274115. PubMed DOI PMC

Wilhelm C., Buchel C., Fisahn J., Goss R., Jakob T., Laroche J., Lavaud J., Lohr M., Riebesell U., Stehfest K., et al. The regulation of carbon and nutrient assimilation in diatoms is significantly different from green algae. Protist. 2006;157:91–124. doi: 10.1016/j.protis.2006.02.003. PubMed DOI

Schoefs B., Hu H., Kroth P.G. The peculiar carbon metabolism in diatoms. Philos. Trans. R. Soc. B Biol. Sci. 2017;372:20160405. doi: 10.1098/rstb.2016.0405. PubMed DOI PMC

Nonoyama T., Kazamia E., Nawaly H., Gao X., Tsuji Y., Matsuda Y., Bowler C., Tanaka T., Dorrell R.G. Metabolic Innovations Underpinning the Origin and Diversification of the Diatom Chloroplast. Biomolecules. 2019;9:322. doi: 10.3390/biom9080322. PubMed DOI PMC

Zrenner R., Stitt M., Sonnewald U., Boldt R. Pyrimidine and purine biosynthesis and degradation in plants. Ann. Rev. Plant Biol. 2006;57:805–836. doi: 10.1146/annurev.arplant.57.032905.105421. PubMed DOI

Garavito M.F., Narváez-Ortiz H.Y., Zimmermann B.H. Pyrimidine Metabolism: Dynamic and Versatile Pathways in Pathogens and Cellular Development. J. Genet. Genom. 2015;42:195–205. doi: 10.1016/j.jgg.2015.04.004. PubMed DOI

Kruger N.J., von Schaewen A. The oxidative pentose phosphate pathway: Structure and organisation. Curr. Opin. Plant Biol. 2003;6:236–246. doi: 10.1016/S1369-5266(03)00039-6. PubMed DOI

Witz S., Jung B., Fürst S., Möhlmann T. De novo pyrimidine nucleotide synthesis mainly occurs outside of plastids, but a previously undiscovered nucleobase importer provides substrates for the essential salvage pathway in Arabidopsis. Plant Cell. 2012;24:1549–1559. doi: 10.1105/tpc.112.096743. PubMed DOI PMC

Michels A.K., Wedel N., Kroth P.G. Diatom plastids possess a phosphoribulokinase with an altered regulation and no oxidative pentose phosphate pathway. Plant. Physiol. 2005;137:911–920. doi: 10.1104/pp.104.055285. PubMed DOI PMC

Kroth P.G., Chiovitti A., Gruber A., Martin-Jezequel V., Mock T., Parker M.S., Stanley M.S., Kaplan A., Caron L., Weber T., et al. A model for carbohydrate metabolism in the diatom Phaeodactylum tricornutum deduced from comparative whole genome analysis. PLoS ONE. 2008;3:e1426. doi: 10.1371/journal.pone.0001426. PubMed DOI PMC

Armbrust E.V., Berges J.A., Bowler C., Green B.R., Martinez D., Putnam N.H., Zhou S., Allen A.E., Apt K.E., Bechner M., et al. The genome of the diatom Thalassiosira pseudonana: Ecology, evolution, and metabolism. Science. 2004;306:79–86. doi: 10.1126/science.1101156. PubMed DOI

Allen A.E., Dupont C.L., Obornik M., Horak A., Nunes-Nesi A., McCrow J.P., Zheng H., Johnson D.A., Hu H., Fernie A.R., et al. Evolution and metabolic significance of the urea cycle in photosynthetic diatoms. Nature. 2011;473:203–207. doi: 10.1038/nature10074. PubMed DOI

Bowler C., Allen A.E., Badger J.H., Grimwood J., Jabbari K., Kuo A., Maheswari U., Martens C., Maumus F., Otillar R.P., et al. The Phaeodactylum genome reveals the evolutionary history of diatom genomes. Nature. 2008;456:239–244. doi: 10.1038/nature07410. PubMed DOI

Gruber A., Vugrinec S., Hempel F., Gould S.B., Maier U.G., Kroth P.G. Protein targeting into complex diatom plastids: Functional characterisation of a specific targeting motif. Plant. Mol. Biol. 2007;64:519–530. doi: 10.1007/s11103-007-9171-x. PubMed DOI

Kilian O., Kroth P.G. Identification and characterization of a new conserved motif within the presequence of proteins targeted into complex diatom plastids. Plant. J. 2005;41:175–183. doi: 10.1111/j.1365-313X.2004.02294.x. PubMed DOI

Ast M., Gruber A., Schmitz-Esser S., Neuhaus H.E., Kroth P.G., Horn M., Haferkamp I. Diatom plastids depend on nucleotide import from the cytosol. Proc. Natl. Acad. Sci. USA. 2009;106:3621–3626. doi: 10.1073/pnas.0808862106. PubMed DOI PMC

Sakaguchi T., Nakajima K., Matsuda Y. Identification of the UMP Synthase Gene by Establishment of Uracil Auxotrophic Mutants and the Phenotypic Complementation System in the Marine Diatom Phaeodactylum tricornutum. Plant Physiol. 2011;156:78–89. doi: 10.1104/pp.110.169631. PubMed DOI PMC

Major P., Embley T.M., Williams T.A. Phylogenetic Diversity of NTT Nucleotide Transport Proteins in Free-Living and Parasitic Bacteria and Eukaryotes. Genome Biol. Evol. 2017;9:480–487. doi: 10.1093/gbe/evx015. PubMed DOI PMC

Chu L., Gruber A., Ast M., Schmitz-Esser S., Altensell J., Neuhaus H.E., Kroth P.G., Haferkamp I. Shuttling of (deoxy-) purine nucleotides between compartments of the diatom Phaeodactylum tricornutum. New Phytol. 2017;213:193–205. doi: 10.1111/nph.14126. PubMed DOI

Schober A.F., Río Bártulos C., Bischoff A., Lepetit B., Gruber A., Kroth P.G. Organelle Studies and Proteome Analyses of Mitochondria and Plastids Fractions from the Diatom Thalassiosira pseudonana. Plant Cell Physiol. 2019;60:1811–1828. doi: 10.1093/pcp/pcz097. PubMed DOI PMC

Nordberg H., Cantor M., Dusheyko S., Hua S., Poliakov A., Shabalov I., Smirnova T., Grigoriev I.V., Dubchak I. The genome portal of the Department of Energy Joint Genome Institute: 2014 updates. Nucleic Acids Res. 2014;42:D26–D31. doi: 10.1093/nar/gkt1069. PubMed DOI PMC

Bendtsen J.D., Nielsen H., von Heijne G., Brunak S. Improved prediction of signal peptides: SignalP 3.0. J. Mol. Biol. 2004;340:783–795. doi: 10.1016/j.jmb.2004.05.028. PubMed DOI

Gruber A., Rocap G., Kroth P.G., Armbrust E.V., Mock T. Plastid proteome prediction for diatoms and other algae with secondary plastids of the red lineage. Plant. J. 2015;81:519–528. doi: 10.1111/tpj.12734. PubMed DOI PMC

Almagro Armenteros J.J., Salvatore M., Emanuelsson O., Winther O., von Heijne G., Elofsson A., Nielsen H. Detecting sequence signals in targeting peptides using deep learning. Life Sci. Alliance. 2019;2:e201900429. doi: 10.26508/lsa.201900429. PubMed DOI PMC

Gasteiger E., Hoogland C., Gattiker A., Duvaud S., Wilkins M.R., Appel R.D., Bairoch A. Protein Identification and Analysis Tools on the ExPASy Server. In: Walker J.M., editor. The Proteomics Protocols Handbook. Humana Press; Totowa, NJ, USA: 2005. pp. 571–607. DOI

Sonnhammer E.L.L., von Heijne G., Krogh A. A hidden Markov model for predicting transmembrane helices in protein sequences. In: Glasgow J., Littlejohn T., Major F., Lathrop R., Sankoff D., Sensen C., editors. Proceedings of the Sixth International Conference on Intelligent Systems for Molecular Biology. AAAI Press; Menlo Park, CA, USA: 1998. pp. 175–182. PubMed

Krogh A., Larsson B., von Heijne G., Sonnhammer E.L.L. Predicting transmembrane protein topology with a hidden markov model: Application to complete genomes11Edited by F. Cohen. J. Mol. Biol. 2001;305:567–580. doi: 10.1006/jmbi.2000.4315. PubMed DOI

Gruber A., Kroth P.G. Deducing intracellular distributions of metabolic pathways from genomic data. Methods Mol. Biol. 2014;1083:187–211. doi: 10.1007/978-1-62703-661-0_12. PubMed DOI

Gschloessl B., Guermeur Y., Cock J.M. HECTAR: A method to predict subcellular targeting in heterokonts. BMC Bioinform. 2008;9:393. doi: 10.1186/1471-2105-9-393. PubMed DOI PMC

Gould S.B., Sommer M.S., Kroth P.G., Gile G.H., Keeling P.J., Maier U.G. Nucleus-to-nucleus gene transfer and protein retargeting into a remnant cytoplasm of cryptophytes and diatoms. Mol. Biol. Evol. 2006;23:2413–2422. doi: 10.1093/molbev/msl113. PubMed DOI

Sommer M.S., Gould S.B., Lehmann P., Gruber A., Przyborski J.M., Maier U.-G. Der1-mediated Preprotein Import into the Periplastid Compartment of Chromalveolates? Mol. Biol. Evol. 2007;24:918–928. doi: 10.1093/molbev/msm008. PubMed DOI

Gruber A., Kroth P.G. Intracellular metabolic pathway distribution in diatoms and tools for genome-enabled experimental diatom research. Philos. Trans. R. Soc. B Biol. Sci. 2017;372:20160402. doi: 10.1098/rstb.2016.0402. PubMed DOI PMC

Mulholland M.R., Rocha A.M., Boneillo G.E. Incorporation of Leucine and Thymidine by Estuarine Phytoplankton: Implications for Bacterial Productivity Estimates. Estuaries Coasts. 2011;34:310–325. doi: 10.1007/s12237-010-9366-2. DOI

Tjaden J., Winkler H.H., Schwöppe C., Van Der Laan M., Möhlmann T., Neuhaus H.E. Two Nucleotide Transport Proteins in Chlamydia trachomatis, One for Net Nucleoside Triphosphate Uptake and the Other for Transport of Energy. J. Bacteriol. 1999;181:1196–1202. PubMed PMC

Haferkamp I., Schmitz-Esser S., Linka N., Urbany C., Collingro A., Wagner M., Horn M., Neuhaus H.E. A candidate NAD+ transporter in an intracellular bacterial symbiont related to Chlamydiae. Nature. 2004;432:622–625. doi: 10.1038/nature03131. PubMed DOI

Heinz E., Hacker C., Dean P., Mifsud J., Goldberg A.V., Williams T.A., Nakjang S., Gregory A., Hirt R.P., Lucocq J.M., et al. Plasma membrane-located purine nucleotide transport proteins are key components for host exploitation by microsporidian intracellular parasites. PLoS Pathog. 2014;10:e1004547. doi: 10.1371/journal.ppat.1004547. PubMed DOI PMC

Dean P., Sendra K.M., Williams T.A., Watson A.K., Major P., Nakjang S., Kozhevnikova E., Goldberg A.V., Kunji E.R.S., Hirt R.P., et al. Transporter gene acquisition and innovation in the evolution of Microsporidia intracellular parasites. Nat. Commun. 2018;9:1709. doi: 10.1038/s41467-018-03923-4. PubMed DOI PMC

Schmitz-Esser S., Linka N., Collingro A., Beier C.L., Neuhaus H.E., Wagner M., Horn M. ATP/ADP Translocases: A Common Feature of Obligate Intracellular Amoebal Symbionts Related to Chlamydiae and Rickettsiae. J. Bacteriol. 2004;186:683–691. doi: 10.1128/JB.186.3.683-691.2004. PubMed DOI PMC

Vávra J., Lukeš J. Chapter Four—Microsporidia and ‘The Art of Living Together’. In: Rollinson D., editor. Advances in Parasitology. Volume 82. Academic Press; San Diego, CA, USA: 2013. pp. 253–319. PubMed DOI

Tsaousis A.D., Kunji E.R.S., Goldberg A.V., Lucocq J.M., Hirt R.P., Embley T.M. A novel route for ATP acquisition by the remnant mitochondria of Encephalitozoon cuniculi. Nature. 2008;453:553–556. doi: 10.1038/nature06903. PubMed DOI

Dean P., Hirt R.P., Embley T.M. Microsporidia: Why Make Nucleotides if You Can Steal Them? PLoS Pathog. 2016;12:e1005870. doi: 10.1371/journal.ppat.1005870. PubMed DOI PMC

Bailleul B., Berne N., Murik O., Petroutsos D., Prihoda J., Tanaka A., Villanova V., Bligny R., Flori S., Falconet D., et al. Energetic coupling between plastids and mitochondria drives CO2 assimilation in diatoms. Nature. 2015;524:366. doi: 10.1038/nature14599. PubMed DOI

Ewe D., Tachibana M., Kikutani S., Gruber A., Bartulos C.R., Konert G., Kaplan A., Matsuda Y., Kroth P.G. The intracellular distribution of inorganic carbon fixing enzymes does not support the presence of a C4 pathway in the diatom Phaeodactylum tricornutum. Photosynth. Res. 2018;137:263–280. doi: 10.1007/s11120-018-0500-5. PubMed DOI

Fabris M., Matthijs M., Rombauts S., Vyverman W., Goossens A., Baart G.J.E. The metabolic blueprint of Phaeodactylum tricornutum reveals a eukaryotic Entner-Doudoroff glycolytic pathway. Plant J. 2012;70:1004–1014. doi: 10.1111/j.1365-313X.2012.04941.x. PubMed DOI

Rio Bartulos C., Rogers M.B., Williams T.A., Gentekaki E., Brinkmann H., Cerff R., Liaud M.-F., Hehl A.B., Yarlett N.R., Gruber A., et al. Mitochondrial Glycolysis in a Major Lineage of Eukaryotes. Genome Biol. Evol. 2018;10:2310–2325. doi: 10.1093/gbe/evy164. PubMed DOI PMC

Ball S.G., Subtil A., Bhattacharya D., Moustafa A., Weber A.P.M., Gehre L., Colleoni C., Arias M.-C., Cenci U., Dauvillée D. Metabolic Effectors Secreted by Bacterial Pathogens: Essential Facilitators of Plastid Endosymbiosis? Plant Cell. 2013;25:7–21. doi: 10.1105/tpc.112.101329. PubMed DOI PMC

Cenci U., Bhattacharya D., Weber A.P.M., Colleoni C., Subtil A., Ball S.G. Biotic Host–Pathogen Interactions As Major Drivers of Plastid Endosymbiosis. Trends Plant Sci. 2017;22:316–328. doi: 10.1016/j.tplants.2016.12.007. PubMed DOI

Dagan T., Roettger M., Stucken K., Landan G., Koch R., Major P., Gould S.B., Goremykin V.V., Rippka R., Tandeau de Marsac N., et al. Genomes of Stigonematalean Cyanobacteria (Subsection V) and the Evolution of Oxygenic Photosynthesis from Prokaryotes to Plastids. Genome Biol. Evol. 2012;5:31–44. doi: 10.1093/gbe/evs117. PubMed DOI PMC

Domman D., Horn M., Embley T.M., Williams T.A. Plastid establishment did not require a chlamydial partner. Nat. Commun. 2015;6:6421. doi: 10.1038/ncomms7421. PubMed DOI PMC

Ball S.G., Bhattacharya D., Qiu H., Weber A.P.M. Commentary: Plastid establishment did not require a chlamydial partner. Front. Cell. Infect. Microbiol. 2016;6 doi: 10.3389/fcimb.2016.00043. PubMed DOI PMC

Haferkamp I., Schmitz-Esser S. The Plant Mitochondrial Carrier Family: Functional and Evolutionary Aspects. Front. Plant Sci. 2012;3 doi: 10.3389/fpls.2012.00002. PubMed DOI PMC

Williams B.A., Haferkamp I., Keeling P.J. An ADP/ATP-specific mitochondrial carrier protein in the microsporidian Antonospora locustae. J. Mol. Biol. 2008;375:1249–1257. doi: 10.1016/j.jmb.2007.11.005. PubMed DOI

Thuswaldner S., Lagerstedt J.O., Rojas-Stütz M., Bouhidel K., Der C., Leborgne-Castel N., Mishra A., Marty F., Schoefs B., Adamska I., et al. Identification, Expression, and Functional Analyses of a Thylakoid ATP/ADP Carrier from Arabidopsis. J. Biol. Chem. 2007;282:8848–8859. doi: 10.1074/jbc.M609130200. PubMed DOI

Gigolashvili T., Geier M., Ashykhmina N., Frerigmann H., Wulfert S., Krueger S., Mugford S.G., Kopriva S., Haferkamp I., Flügge U.-I. The Arabidopsis thylakoid ADP/ATP carrier TAAC has an additional role in supplying plastidic phosphoadenosine 5′-phosphosulfate to the cytosol. Plant Cell. 2012;24:4187–4204. doi: 10.1105/tpc.112.101964. PubMed DOI PMC

Klein M.-C., Zimmermann K., Schorr S., Landini M., Klemens P.A.W., Altensell J., Jung M., Krause E., Nguyen D., Helms V., et al. AXER is an ATP/ADP exchanger in the membrane of the endoplasmic reticulum. Nat. Comm. 2018;9:3489. doi: 10.1038/s41467-018-06003-9. PubMed DOI PMC

Major P., Sendra K.M., Dean P., Williams T.A., Watson A.K., Thwaites D.T., Embley T.M., Hirt R.P. A new family of cell surface located purine transporters in Microsporidia and related fungal endoparasites. eLife. 2019;8:e47037. doi: 10.7554/eLife.47037. PubMed DOI PMC

Cavalier-Smith T. Principles of protein and lipid targeting in secondary symbiogenesis: Euglenoid, dinoflagellate, and sporozoan plastid origins and the eukaryote family tree. J. Eukaryot. Microbiol. 1999;46:347–366. doi: 10.1111/j.1550-7408.1999.tb04614.x. PubMed DOI

Petersen J., Ludewig A.K., Michael V., Bunk B., Jarek M., Baurain D., Brinkmann H. Chromera velia, endosymbioses and the rhodoplex hypothesis--plastid evolution in cryptophytes, alveolates, stramenopiles, and haptophytes (CASH lineages) Genome Biol. Evol. 2014;6:666–684. doi: 10.1093/gbe/evu043. PubMed DOI PMC

Derelle R., López-García P., Timpano H., Moreira D. A Phylogenomic Framework to Study the Diversity and Evolution of Stramenopiles (=Heterokonts) Mol. Biol. Evol. 2016;33:2890–2898. doi: 10.1093/molbev/msw168. PubMed DOI PMC

Malyshev D.A., Dhami K., Lavergne T., Chen T., Dai N., Foster J.M., Correa I.R., Jr., Romesberg F.E. A semi-synthetic organism with an expanded genetic alphabet. Nature. 2014;509:385–388. doi: 10.1038/nature13314. PubMed DOI PMC

Dien V.T., Morris S.E., Karadeema R.J., Romesberg F.E. Expansion of the genetic code via expansion of the genetic alphabet. Curr. Opin. Chem. Biol. 2018;46:196–202. doi: 10.1016/j.cbpa.2018.08.009. PubMed DOI PMC

Eremeeva E., Herdewijn P. Non canonical genetic material. Curr. Opin. Biotechnol. 2019;57:25–33. doi: 10.1016/j.copbio.2018.12.001. PubMed DOI

Zhang Y., Lamb B.M., Feldman A.W., Zhou A.X., Lavergne T., Li L., Romesberg F.E. A semisynthetic organism engineered for the stable expansion of the genetic alphabet. Proc. Natl. Acad. Sci. USA. 2017;114:1317–1322. doi: 10.1073/pnas.1616443114. PubMed DOI PMC

Zhang Y., Ptacin J.L., Fischer E.C., Aerni H.R., Caffaro C.E., San Jose K., Feldman A.W., Turner C.R., Romesberg F.E. A semi-synthetic organism that stores and retrieves increased genetic information. Nature. 2017;551:644. doi: 10.1038/nature24659. PubMed DOI PMC

Feldman A.W., Fischer E.C., Ledbetter M.P., Liao J.-Y., Chaput J.C., Romesberg F.E. A Tool for the Import of Natural and Unnatural Nucleoside Triphosphates into Bacteria. J. Am. Chem. Soc. 2018;140:1447–1454. doi: 10.1021/jacs.7b11404. PubMed DOI PMC

Pezo V., Hassan C., Louis D., Sargueil B., Herdewijn P., Marlière P. Metabolic Recruitment and Directed Evolution of Nucleoside Triphosphate Uptake in Escherichia coli. ACS Synth. Biol. 2018;7:1565–1572. doi: 10.1021/acssynbio.8b00048. PubMed DOI

Huang W., Daboussi F. Genetic and metabolic engineering in diatoms. Philos. Trans. R. Soc. B Biol. Sci. 2017;372:20160411. doi: 10.1098/rstb.2016.0411. PubMed DOI PMC

Evolutionary and Molecular Aspects of Plastid Endosymbioses