*Department of Microbiology and Evolutionary Biology, Faculty of Science, University of Nijmegen, Nijmegen, The Netherlands;
and
Biocomputing EMBL, Heidelberg, Germany, Max-Delbrück-Centrum for Molecular Medicine, Berlin-Buch, Germany, and Bioinformatics Group, University of Utrecht, Utrecht, The Netherlands
Hydrogenosomes are membrane-bounded organelles about 1 µm 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, sincewith one notable exceptionall 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 double-stranded 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 transcriptionpolymerase 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 (vanHoek et al. 1998, 1999, 2000
). Notwithstanding a high degree of sequence divergence, secondary-structure analysis clearly revealed that all stem/loop structures characteristic of the "structural cores" of the SSU rRNAs (Stiegler et al. 1981
; Gray, Sankoff, and Cedergren 1984
) were conserved. Phylogenetic analysis based on evolutionary conserved rDNA sequence blocks (universally conserved regions U1U8; Gray, Sankoff, and Cedergren 1984
) revealed that the isolated sequence clustered together with mitochondrial SSU rRNA genes of aerobic ciliates (Akhmanova et al. 1998
).
We showed earlier that the ciliates hosted by different cockroach strains exhibit substantial sequence divergence of their nuclear ribosomal genes. The degree of this sequence divergence (up to 9%), the presence of host-specific methanogenic endosymbionts, and differences in morphology and swimming behavior suggest that different cockroach strains host different (sub)species of N. ovalis (van Hoek et al. 1998, 1999, 2000
). Here, we report the isolation of additional hydrogenosomal SSU rRNA genes of different N. ovalis isolates from several cockroach strains. In addition to the previously described complete hydrogenosomal SSU rRNA sequence of N. ovalis from Periplaneta americana var. Nijmegen, we isolated partial SSU rRNA sequences from ciliates hosted by the strains P. americana var. Bayer and P. americana var. Amsterdam. These sequences display substantial DNA sequence divergence, and ciliates from the host strain P. americana var. Amsterdam exhibit heteroplasmy (cf. Lightowlers et al. 1997
). The SSU rRNA sequences Amsterdam 1 and Amsterdam 2 differ in the nonconserved regions of the fragment for several nucleotides. However, an alignment of conserved blocks of hydrogenosomal, mitochondrial, and bacterial SSU rRNAs is possible (fig. 1
). The alignment allows a phylogenetic reconstruction that is supported by several methods of data analysis. Notably, phylogenetic analysis reveals monophyly of the SSU rRNAs from hydrogenosomes and mitochondria from ciliates (fig. 2a
). This would be expected if hydrogenosomes of anaerobic ciliates evolved from the mitochondria of their aerobic relatives.
|
|
We do not yet know whether the putative genome is circular or linear. Assuming a genome of 11 kb or larger (if circular), then the hydrogenosomal genome might be well within the size range of mitochondrial genomes (Gray, Burger, and Lang 1999
). The most related (linear) mitochondrial genomes of the aerobic ciliates Paramecium and Tetrahymena measure approximately 4050 kb (Nosek et al. 1998
; Gray, Burger, and Lang 1999
). We have not yet proven that the putative hydrogenosomal SSU rRNA genes are transcribed inside the hydrogenosomes or that the corresponding rRNAs are actually used for hydrogenosomal protein synthesis. However, to our knowledge, there is not a single report on the presence of (transcribed) mitochondrial ribosomal genes outside a mitochondrion in any eukaryote whatsoever. Moreover, immunogold labeling using antibodies against double-stranded DNA has revealed the presence of DNA in the macronucleus and the micronucleus of the ciliate, the methanogenic endosymbionts, and the mitochondria-like hydrogenosomes (Akhmanova et al. 1998
; unpublished data). Since the cytoplasm is virtually free of label, it is reasonable to assume that the "mitochondrial" SSU rDNA is located in the mitochondria-like hydrogenosomes. Therefore, we have to conclude that the hydrogenosomes of N. ovalis evolved from mitochondriamost likely in a process that involved adaptation of the ciliates to anaerobic environments (Embley et al. 1995
; Hirt, Wilkinson, and Embley 1998
).
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Acknowledgements |
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Footnotes |
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1 Keywords: hydrogenosomes,
mitochondria,
evolution,
anaerobic ciliates,
ribosomal small-subunit RNA (SSU rRNA).
2 Present address: DLO State Institute for Quality Control of Agricultural Products (RIKILT), Wageningen, The Netherlands.
3 Present address: Department of Cell Biology and Genetics, Erasmus University, Rotterdam, The Netherlands.
4 Address for correspondence and reprints: J. H. P. Hackstein, Department of Microbiology and Evolutionary Biology, Faculty of Science, University of Nijmegen, Toernooiveld, NL-6525 ED Nijmegen, The Netherlands. E-mail: hack{at}sci.kun.nl
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References |
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