Article: Evidence for organelle origin

From: Robert Karl Stonjek (rstonjek_at_bigpond.net.au)
Date: 03/04/05 

Date: Fri, 4 Mar 2005 01:15:06 -0500 (EST)

Evidence for organelle origin
Genetic clues point to 'missing links' between hydrogenosomes and
mitochondria
By Charles Q Choi

The first genetic evidence of hydrogenosomes-organelles found in some
ciliates, trichomonads, and fungi that generate hydrogen and adenosine
triphosphate (ATP)-that evolved from mitochondria is reported by Dutch and
German researchers in Nature this week.

"Our findings prove the existence of missing links between mitochondria and
hydrogenosomes," coauthor Johannes Hackstein of Radboud University Nijmegen
in the Netherlands told The Scientist. "This has value in analyzing the
evolution of the eukaryotic cell, to help better understand these organelles
and possibly mitochondrial dysfunctions."

Hydrogenosomes resemble mitochondria morphologically, but generally lack a
genome, hampering clarification of their origin. Considerable debate exists
over whether hydrogenosomes evolved from aerobically functioning
mitochondria that acquired an anaerobic metabolism or from endosymbionts
capable of both aerobic and anaerobic function that mitochondria also
descended from.

In 1998, Hackstein and colleagues found preliminary evidence that
hydrogenosomes of the anaerobic ciliate Nyctotherus ovalis from cockroach
guts might possess genes. In the research team's latest study, long-range
polymerase chain reaction (PCR) on N. ovalis hydrogenosome DNA yielded a
12-kilobase fragment of the organellar genome that encodes four genes of a
mitochondrial complex I (nad2, nad4L, nad5, and nad7), two genes encoding
mitochondrial ribosomal proteins (RPL2 and RPL14), and a tRNA tyrosine gene.
The genes exhibit distinct mitochondrial codon usage, and complementary DNAs
isolated for nad5 and nad7 show they are transcribed.

Investigating N. ovalis, the researchers identified three more mitochondrial
I complex genes in its nucleus, for a total of seven of the 14 core genes
needed for that complex. Hackstein and colleagues also found genes for half
of the proteins of mitochondrial complex II, the Fp and Ip subunits, in the
nucleus. Phylogenetic analysis with a discrete gamma-distribution model
revealed that both hydrogenosomal and nuclear genes appear well conserved
and that they cluster with their homologues from the mitochondrial genomes
of aerobic ciliates.

The N. ovalis hydrogenase's catalytic center clusters neither with
hydrogenases from eukaryotes nor with any of the hydrogenase-related Nar
proteins that seem shared by all eukaryotes. Rather, the N. ovalis
hydrogenase is more closely related to delta proteobacterial [Fe]
hydrogenases. "This implies that the hydrogenase was acquired by lateral
gene transfer," Hackstein said, adding weight to the idea the organelle was
a mitochondrion that became a hydrogenosome secondarily.

Full text at TheScientist
http://www.biomedcentral.com/news/20050303/01

Posted by
Robert Karl Stonjek