The hydrogenosomes of Psalteriomonas lanterna

Abstract

Background: Hydrogenosomes are organelles that produce molecular hydrogen and ATP. The broad phylogenetic distribution of their hosts suggests that the hydrogenosomes of these organisms evolved several times independently from the mitochondria of aerobic progenitors. Morphology and 18S rRNA phylogeny suggest that the microaerophilic amoeboflagellate Psalteriomonas lanterna, which possesses hydrogenosomes and elusive "modified mitochondria", belongs to the Heterolobosea, a taxon that consists predominantly of aerobic, mitochondriate organisms. This taxon is rather unrelated to taxa with hitherto studied hydrogenosomes.

Results: Electron microscopy of P. lanterna flagellates reveals a large globule in the centre of the cell that is build up from stacks of some 20 individual hydrogenosomes. The individual hydrogenosomes are surrounded by a double membrane that encloses a homogeneous, dark staining matrix lacking cristae. The "modified mitochondria" are found in the cytoplasm of the cell and are surrounded by 1-2 cisterns of rough endoplasmatic reticulum, just as the mitochondria of certain related aerobic Heterolobosea. The ultrastructure of the "modified mitochondria" and hydrogenosomes is very similar, and they have the same size distribution as the hydrogenosomes that form the central stack.The phylogenetic analysis of selected EST sequences (Hsp60, Propionyl-CoA carboxylase) supports the phylogenetic position of P. lanterna close to aerobic Heterolobosea (Naegleria gruberi). Moreover, this analysis also confirms the identity of several mitochondrial or hydrogenosomal key-genes encoding proteins such as a Hsp60, a pyruvate:ferredoxin oxidoreductase, a putative ADP/ATP carrier, a mitochondrial complex I subunit (51 KDa), and a [FeFe] hydrogenase.

Conclusion: Comparison of the ultrastructure of the "modified mitochondria" and hydrogenosomes strongly suggests that both organelles are just two morphs of the same organelle. The EST studies suggest that the hydrogenosomes of P. lanterna are physiologically similar to the hydrogenosomes of Trichomonas vaginalis and Trimastix pyriformis. Phylogenetic analysis of the ESTs confirms the relationship of P. lanterna with its aerobic relative, the heterolobosean amoeboflagellate Naegleria gruberi, corroborating the evolution of hydrogenosomes from a common, mitochondriate ancestor.

Figure 1

Light microscopy of Psalteriomonas lanterna. A: Flagellate stage of Psalteriomonas lanterna DIC-microscopy. At the apical side of the cell two of the four flagella clusters can be seen. The globule in the centre of the cell is the hydrogenosomal complex. B: Amoeba stage of Psalteriomonas lanterna. CLS-microscope. Bars: 30 μm

Figure 2

Electron microscopy of the hydrogenosomes of Psalteriomonas lanterna flagellates. A: Cell with two small stacks of hydrogenosomes. HC: hydrogenosomal complex. B: Group of dumb-bell-shaped hydrogenosomes in the periphery of the cell. The hydrogenosomes are surrounded by cisterns of rough endoplasmatic reticulum (rough ER). These organelles have been named "modified mitochondria" by Broers (1992) [25]. C: Large stack of hydrogenosomes (HC). D: Detail of the hydrogenosomal complex shown in C. E: "Single" hydrogenosome surrounded by rough ER. F: Dumb-bell-shaped hydrogenosome ("modified mitochondrion"). Bars A-D, F: 1 μm; E: 0,5 μm

Figure 3

Histogram of the lengths of hydrogenosomes. Randomly selected sections of hydrogenosomes on the electron micrographs were measured and plotted. "dividing": dumb-bell-shaped organelles. "complexes": hydrogenosomes from stacks. All kinds of hydrogenosomes belong to the same length distribution.

Figure 4

ML tree for the Elongation Factor 1 alpha. This tree was computed using the RtREV+4 discrete-rate G+I+F. The tree is rooted by an outgroup of Archaeal species (in blue). Branch values represent the bootstrap percentage.

Figure 5

Phylogeny of the Heat Shock Protein 60. The branch values represent bootstrap values. ML tree computed with RtREV+4 discrete-rate G+I+F. An outgroup of Bacteria was chosen to root this tree (in blue).

Figure 6

ML phylogeny of the putative ADP/ATP carrier (member of the Mitochondrial Solute Carrier family). Branch values are the bootstrap percentages, and the tree was computed using a RtREV+4 discrete-rate G+F.

Figure 7

Phylogeny of the [FeFe]Hydrogenase, based on the alignment of the H-cluster. ML bootstrap support values are indicated in the branches. ML computation using a WAG+4 discrete-rate G+I+F model.

Figure 8

Phylogenetic tree of the Pyruvate:ferredoxin oxidoreductase, computed by ML with WAG+4 discrete-rate G+I+F. Branch support values represent bootstrap values.

Figure 9

ML phylogenetic tree of Propionyl Co-A carboxylase (PCCB). Branch values are bootstrap support. This tree is rooted by 2 archaeal species, and was computed using a WAG+4 discrete-rate G+I model.

Figure 10

NADH:ubiquinone oxidoreductase 51 kDa subunit ML phylogeny with bootstrap values indicated in the branches, and computed using a WAG+4 discrete-rate G.

Figure 11

Phylogenetic tree of the Glutamate dehydrogenase computed by ML with a RtREV+4 discrete-rate G+I+F. Branches show the bootstrap support values.

Figure 12

Rudimentary metabolic scheme of the hydrogenosomes of P. lanterna.