Sequence and analysis of chromosome I of the amitochondriate intracellularparasite Encephalitozoon cuniculi (Microspora)

A DNA sequencing program was applied to the small (<3 Mb) genome of the mic rosporidian Encephalitozoon cuniculi, an amitochondriate eukaryotic parasit e of mammals, and the sequence of the smallest chromosome was determined. T he <similar to>224-kb E. cuniculi chromosome I exhibits a dyad symmetry cha racterized by two identical 37-kb subtelomeric regions which are divergentl y oriented and extend just downstream of the inverted copies of an 8-kb dup licated cluster of six genes. Each subtelomeric region comprises a single 1 65-235 rDNA transcription unit, flanked by various tandemly repeated sequen ces, and ends with similar to1 kb of heterogeneous telomeric repeats. The c entral (or core) region of the chromosome harbors a highly compact arrangem ent of 132 potential protein-coding genes plus two tRNA genes (one gene per 1.14 kb). Most genes occur as single copies with no identified introns. Of these putative genes, only 53 could be assigned to known functions. A numb er of genes from the transcription and translation machineries as well as f rom other cellular processes display characteristic eukaryotic signatures o r are clearly eukaryote-specific.