ABSTRACTS:
TITLE: ORGANELLE GENOMES: GOING, GOING, GONE!
AUTHOR: JEFFREY PALMER
JOURNAL: SCIENCE, VOL. 275, 2/7/97 [AXX158]
ABSTRACT: Mitochondria and chloroplasts are what is known as endosymbionts. These organelles have persisted within eukaryotic cells for anywhere from 1 - 2 billion years, and have lost most of their genes. Yet all chloroplasts and respiring mitochondria retain a functional genome of at least five genes. Now, however, a eukaryotic organelle - the hydrogenosome - has been identified as endosymbiotic in origin, yet it lacks a genome and is entirely dependent on the nucleus for its genetic livelihood.
Although hydrogenosome occurs widely, it is only in trichomonads that its origins have been clearly determined. Trichomonads are largely parasitic, flagellated protists. These air-tolerant anaerobes lack mitochondria. Instead, they use unusual energy-generating hydrogenosomes. Recent studies indicate that the hydrogenosome is actually a highly derived mitochondrion. Like mitochondria, trichomonad hydrogenosomes have a double-membrane envelope, divide autonomously by fission, import proteins post-translationally, and produce ATP by substrate-level phosphorylation. They differ from mitochondria in that they lack a genome, cytochromes, a tricarboxylic acid cycle, and oxidative phosphorylation. Instead, they use enzymes typically used by anaerobes and produce large quantities of hydrogen.
Two hypotheses have been suggested to explain the origin of hydrogenosomes. It may be the result of an independent endosymbiosis of an anaerobic eubacterium or a highly modified mitochondrion adapted to an anaerobic lifestyle. The fact that the nucleus of Trichomonas contains genes for heat shock proteins (Hsp) suggests that the latter theory may be correct. These Hsps are the most reliable indicators of the eubacterial ancestry of both the mitochondrion and chloroplast and suggest a common origin for the mitochondrion and hydrogenosome. More than likely the hydrogenosome is a highly derived mitochondrion.
Hydrogenosomes are an example of the repeated evolution of biochemically similar organelles. Hydrogenosomes can be found in a wide variety of otherwise unrelated anaerobic or microaerobic eukaryotes, almost all of which lack mitochondria.. These findings should prompt renewed inquiry into the early evolution of the eukaryotic cell. The prevailing view has been that the mitochondrion is not an ancestral feature of the eukaryotic cell. We know that genomic extinction has happened repeatedly for the cell nuclei in many cases where protists engulf eukaryotic alga and permanently retain part of the prey as a degenerate endosymbiont. The host nucleus seems to be a magnet for organellar genes and for endosymbiotic nuclear genes.
KEY WORDS/PHRASES
ANAEROBES
ATP
CYTOCHROMES
DEGENERATE
ENDOSYMBIOSIS
ENDOSYMBIONT
EUBACTERIUM
EUKARYOTES
FLAGELLATED PROTISTS
GENOMIC EXTINCTION
HEAT SHOCK PROTEINS
HYDROGENOSOME
MITOCHONDRIA
ORGANELLAR GENES
PARASITIC
TRICARBOXYLIC ACID CYCLE
TRICHOMONADS
PHOSPHORYLATION
TITLE: A PROTEIN-COUNTING MECHANISM FOR TELOMERE LENGTH REGULATION IN YEAST
AUTHOR: STEPHANE MARCAND, ET AL.
JOURNAL: SCIENCE, VOL. 275, 2/14/97 [AXX164]
ABSTRACT: Telomeres, the ends of linear eukaryotic chromosomes, are essential structures formed by specific protein-DNA complexes that protect chromosomal termini from degradation and fusion. The progressive loss of DNA that would occur after each round of replication is balanced by a ribonucleoprotein terminal transferase enzyme called telomerase, which specifically extends the 3' G-rich telomeric strand in an RNA-templated reaction. In yeast, telomeric DNA is organized in a nonnucleosomal structure based on an array of the telomere repeat-binding protein Rap1p.
In human germ line cells, telomerase is the result of gene (DNA) expression and a constant average telomere length is maintained. As to somatic cells, the initial size of the telomere seems to regulate the life span of the cell since with each replication of the chromosome, the telomere undergoes shortening (or a reduction) in the number of sequence repeats. Telomerase activity is normally undetectable in such cells.
In unicellular organisms such as the yeast Saccharomyces cerevisiae, telomere length is kept within a narrow size distribution that is specific for each strain and species. The process of telomere regulation in such organisms can be viewed as a balance between telomere shortening and elongation. The fact that telomere length is so consistently preserved suggests that a mechanism must exist to sense the length of the telomere. In an experiment designed to test this hypothesis, Rap1p molecules (known to be involved in yeast telomere regulation) were added to yeast telomeres. When these molecules were added to one side of a telomere, it resulted in an equivalent loss on the other side, suggesting that the number of Rap1p molecules is actively maintained at a relatively constant value. This may happen through simple feedback where a telomere is bound by a threshold number of Rap1p molecules (or more), keeping in a state which prevents elongation. When the chromosome degrades, it loses some Rap1p binding sites, which allows it to begin elongating. The process of elongation opens new Rap1p binding sites which permits it to be bound by additional Rap1p molecules and its elongation halted.
Confirmation of this thesis also comes from experiments with a related yeast, Kluyveromyces lactis. Induced mutations in the yeast's telomerase RNA template, which alters telomeric DNA sequences and reduces K.lactis telomerase binding, result in massive telomere elongation. If telomerase cannot generate Rap1p -binding sites, a threshold number of bound Rap1p is never reached and telomere elongation continues without limitation.
KEY WORDS/PHRASES
GENE (DNA) EXPRESSION
GERM LINE CELLS
SOMATIC CELLS
TELOMERE