Abstract 207 10.1 Introduction 208 10.2 The Fascinating Microsporidian World 208 10.3 Why Study Microsporidian Genomes? 210 10.4 E. cuniculi Genome Content 212 A Tiny Microsporidian Genome 212 10.4.2 The Relation of One Chromosomal End – One 16S-23S rDNA Unit 213 10.4.3 A Reduced Set of tRNA Genes 215 10.4.4 The High Protein-Coding Gene Density of the Chromosome Cores 215 10.4.5 Gene Duplications 217 10.4.6 Preservation of Essential Cellular Processes vs. Metabolic

Simplification 219 10.5 Microsporidial Genes Encoding Mitochondrial-Type Proteins 222

10.5.1 Pyruvate Fate and NADH Reoxidation: Two Major Uncertainties 223 10.5.2 A Putative Organellar Protein Import System 224 10.5.3 Iron-Sulfur Cluster Assembly: A Ubiquitous Function for the

E. Cuniculi Mitosome 226 10.5.4First Experimental Evidence of the Microsporidial Mitosome

and Related Questions 227 10.6 Conclusions 228 Acknowledgments 228 References 229

Encephalitozoon cuniculi, a parasite of humans and other mammals, belongs to a large group of amitochondriate unicellular eukaryotes called microsporidia, which conjugate a strict intracellular parasitic lifestyle to the capacity of survival in the environment as resistant spores. Long classified as Protozoa, microsporidia are currently viewed as fungi-related organisms. Illustrating an extreme case of genomic compaction among eukaryotic microbes, the 2.9-Mbp genome of E. cuniculi composed of 11 linear chromosomes has been sequenced. We discuss the contribution that the analysis of the E. cuniculi genome content has made to our knowledge of the biology of eukaryotic parasites. The energy metabolism of E.