Microsporidia constitute a group of extremely specialized intracellular parasites that infect virtually all animals. They are highly derived, reduced fungi that lack several features typical of other eukaryotes, including canonical mitochondria, flagella, and peroxisomes. Consistent with the absence of peroxisomes in microsporidia, the recently completed genome of the microsporidian Encephalitozoon cuniculi lacks a gene for catalase, the major enzymatic marker for the organelle. We show, however, that the genome of the microsporidian Nosema locustae, in contrast to that of E. cuniculi, encodes a group II large-subunit catalase. Surprisingly, phylogenetic analyses indicate that the N. locustae catalase is not specifically related to fungal homologs, as one would expect, but is instead closely related to proteobacterial sequences. This finding indicates that the N. locustae catalase is derived by lateral gene transfer from a bacterium. The catalase gene is adjacent to a large region of the genome that appears to be far less compact than is typical of microsporidian genomes, a characteristic which may make this region more amenable to the insertion of foreign genes. The N. locustae catalase gene is expressed in spores, and the protein is detectable by Western blotting. This type of catalase is a particularly robust enzyme that has been shown to function in dormant cells, indicating that the N. locustae catalase may play some functional role in the spore. There is no evidence that the N. locustae catalase functions in a cryptic peroxisome.
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