Functional characterization of the Chlamydomonas reinhardtii ERG3 ortholog, a gene involved in the biosynthesis of ergosterol.
BACKGROUND
The predominant sterol in the membranes of the alga Chlamydomonas reinhardtii is ergosterol, which is commonly found in the membranes of fungi, but is rarely found in higher plants. Higher plants and fungi synthesize sterols by different pathways, with plants producing cycloartenol as a precursor to end-product sterols, while non-photosynthesizing organisms like yeast and humans produce lanosterol as a precursor. Analysis of the C. reinhardtii genome sequence reveals that this algae is also likely to synthesize sterols using a pathway resembling the higher plant pathway, indicating that its sterols are synthesized somewhat differently than in fungi. The work presented here seeks to establish experimental evidence to support the annotated molecular function of one of the sterol biosynthetic genes in the Chlamydomonas genome.


RESULTS
A gene with homology to the yeast sterol C-5 desaturase, ERG3, is present in the Chlamydomonas genome. To test whether the ERG3 ortholog of C. reinhardtii encodes a sterol C-5 desaturase, Saccharomyces cerevisiae ERG3 knockout strains were created and complemented with a plasmid expressing the Chlamydomonas ERG3. Expression of C. reinhardtii ERG3 cDNA in erg3 null yeast was able to restore ergosterol biosynthesis and reverse phenotypes associated with lack of ERG3 function.


CONCLUSIONS
Complementation of the yeast erg3 null phenotypes strongly suggests that the gene annotated as ERG3 in C. reinhardtii functions as a sterol C-5 desaturase.
Brumfield
Kristy M
KM
Department of Biology, Xavier University of Louisiana, New Orleans, Louisiana, USA.
Moroney
James V
JV
Moore
Thomas S
TS
Simms
Tiffany A
TA
Donze
David
D
D000595:Amino Acid Sequence; D016825:Chlamydomonas reinhardtii / Q000201:enzymology / Q000235:genetics*; D004875:Ergosterol / Q000096:biosynthesis*; D005816:Genetic Complementation Test; D008969:Molecular Sequence Data; D009154:Mutation; D010088:Oxidoreductases / Q000737:chemistry / Q000378:metabolism*; D011995:Recombination, Genetic; D017386:Sequence Homology, Amino Acid
D010088:Oxidoreductases; C039306:sterol delta-5 desaturase; D004875:Ergosterol