A mitochondrial ancestry of the hydrogenosomes of Nyctotherus ovalis.

Hydrogenosomes are membrane-bounded organelles about 1 mm in size that, like mitochondria, produce ATP (Müller 1993). They compartmentalize terminal steps of anaerobic energy metabolism but, unlike mitochondria, hydrogenosomes cannot use oxygen as an electron acceptor; they reduce protons to molecular hydrogen (Embley and Martin 1998; Martin and Müller 1998; Müller 1998). Hydrogenosomes have been found only in anaerobic protists and fungi, for example, the parabasalian flagellate Trichomonas vaginalis, the amoeboflagellate Psalteriomonas lanterna, the anaerobic ciliate Nyctotherus ovalis, and the anaerobic chytridiomycete fungi Neocallimastix frontalis and Piromyces sp. (Müller 1993). A wealth of biochemical and molecular genetic evidence argues that hydrogenosomes share a common ancestor with mitochondria (Biagini, Finlay, and Lloyd 1997; Embley, Horner, and Hirt 1997; Sogin 1997; Martin and Müller 1998). However, since—with one notable exception—all hydrogenosomes studied so far lack a genome, a direct proof remained difficult. Recently, we provided evidence for the presence of a genome in the hydrogenosomes of the gut-dwelling anaerobic ciliate N. ovalis (Akhmanova et al. 1998). Immunogold labeling allowed colocalization of hydrogenase and doublestranded DNA in the hydrogenosomes of the ciliate. These organelles look like mitochondria, but they are surrounded by endosymbiotic methanogenic archaea that indicate (1) anoxic conditions in the immediate vicinity of the mitochondria-like organelles and (2) the presence of intracellular hydrogen (cf. Fenchel and Finlay 1995). Using reverse transcription–polymerase chain reaction (RT-PCR) and PCR approaches, we succeeded in isolating and sequencing a complete SSU rRNA that is abundantly expressed in the ciliates (Akhmanova et al. 1998). This putative hydrogenosomal SSU rRNA (accession number Y16670) proved to be greatly different with regard to nucleotide sequence and the lengths of variable regions from the respective SSU rRNA genes of the ciliate and its methanogenic endosymbionts (van Hoek et al. 1998, 1999, 2000). Notwithstanding a high