MICROSPORIDIA, AMITOCHONDRIAL PROTISTS, POSSESS A 70-KDA HEAT-SHOCK-PROTEIN GENE OF MITOCHONDRIAL EVOLUTIONARY ORIGIN

An intronless gene encoding a protein of 592 amino acid residues with similarity to 70-kDa heat shock proteins (HSP70s) has been cloned and sequenced from the amitochondrial protist Encephalitozoon cuniculi (ph ylum Microsporidia). Southern blot analyses show the presence of a sin gle gene copy located on chromosome XI. The encoded protein exhibits a n N-terminal hydrophobic leader sequence and two motifs shared by prot eobacterial and mitochondrially expressed HSP70 homologs. Phylogenetic analysis using maximum Likelihood and evolutionary distances place th e E. cuniculi sequence in the cluster of mitochondrially expressed HSP 70s, with a higher evolutionary rate than those of homologous sequence s. Similar results were obtained after cloning a fragment of the homol ogous gene in the closely related species E. hellem. The presence of a nuclear targeting signal-like sequence supports a role of the Encepha litozoon HSP70 as a molecular chaperone of nuclear proteins. No eviden ce for cytosolic or endoplasmic reticulum forms of HSP70 was obtained through PCR amplification. These data suggest that Encephalitozoon spe cies have evolved from an ancestor bearing mitochondria, which is in d isagreement with the postulated presymbiotic origin of Microsporidia. The specific role and intracellular localization of the mitochondrial HSP70-like protein remain to be elucidated.