Affiliation:
1. Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada
Abstract
ABSTRACT
The presence of plastids in diverse eukaryotic lineages that have lost the capacity for photosynthesis is well documented. The metabolic functions of such organelles, however, are poorly understood except in the case of the apicoplast in the Apicomplexa, a group of intracellular parasites including
Plasmodium falciparum
, and the plastid of the green alga
Helicosporidium
sp., a parasite for which the only host-free stage identified in nature so far is represented by cysts. As a first step in the reconstruction of plastid functions in a nonphotosynthetic, predominantly free-living organism, we searched for expressed sequence tags (ESTs) that correspond to nucleus-encoded plastid-targeted polypeptides in the green alga
Prototheca wickerhamii
. From 3,856 ESTs, we found that 71 unique sequences (235 ESTs) correspond to different nucleus-encoded putatively plastid-targeted polypeptides. The identified proteins predict that carbohydrate, amino acid, lipid, tetrapyrrole, and isoprenoid metabolism as well as de novo purine biosynthesis and oxidoreductive processes take place in the plastid of
P. wickerhamii
. Mg-protoporphyrin accumulation and, therefore, plastid-to-nucleus signaling might also occur in this nonphotosynthetic organism, as we identified a transcript which encodes subunit I of Mg-chelatase, the enzyme which catalyzes the first committed step in chlorophyll synthesis. Our data indicate a far more complex metabolism in
P. wickerhamii
's plastid compared with the metabolic pathways predicted to be located in the apicoplast of
P. falciparum
and the plastid of
Helicosporidium
sp.
Publisher
American Society for Microbiology
Subject
Molecular Biology,General Medicine,Microbiology
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