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20 enero 2010

Chloroplast genes are expressed during intracellular symbiotic association of Vaucheria litorea plastids with the sea slug Elysia chlorotica.


C V Mujer, D L Andrews, J R Manhart, S K Pierce, and M E Rumpho
Department of Horticultural Sciences, Texas A & M University, College
Station 77843, USA.
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The marine slug Elysia chlorotica (Gould) forms an intracellular symbiosis
with photosynthetically active chloroplasts from the chromophytic alga
Vaucheria litorea (C. Agardh). This symbiotic association was characterized
over a period of 8 months during which E. chlorotica was deprived of V.
litorea but provided with light and CO2. The fine structure of the symbiotic
chloroplasts remained intact in E. chlorotica even after 8 months of
starvation as revealed by electron microscopy. Southern blot analysis of
total DNA from E. chlorotica indicated that algal genes, i.e., rbcL, rbcS,
psaB, psbA, and 16S rRNA are present in the animal. These genes are
typically localized to the plastid genome in higher plants and algae except
rbcS, which is nuclear-encoded in higher plants and green (chlorophyll a/b)
algae. Our analysis suggests, however, that similar to the few other
chromophytes (chlorophyll a/c) examined, rbcS is chloroplast encoded in V.
litorea. Levels of psbA transcripts remained constant in E. chlorotica
starved for 2 and 3 months and then gradually declined over the next 5
months corresponding with senescence of the animal in culture and in nature.
The RNA synthesis inhibitor 6-methylpurine reduced the accumulation of psbA
transcripts confirming active transcription. In contrast to psbA, levels of
16S rRNA transcripts remained constant throughout the starvation period. The
levels of the photosystem II proteins, D1 and CP43, were high at 2 and 4
months of starvation and remained constant at a lower steady-state level
after 6 months. In contrast, D2 protein levels, although high at 2 and 4
months, were very low at all other periods of starvation. At 8 months, de
novo synthesis of several thylakoid membrane-enriched proteins, including
D1, still occurred. To our knowledge, these results represent the first
molecular evidence for active transcription and translation of algal
chloroplast genes in an animal host and are discussed in relation to the
endosymbiotic theory of eukaryote origins.

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