Chloroplast symbiosis: organization and expression of chloroplast genes and maintenance of chloroplast activity in a foreign host

Abstract

The observation of maintenance of photosynthesis by endosymbiotic Vaucheria litorea chloroplasts in the foreign mollusc host Elysia chlorotica over at least seven months indicates a functional genetic apparatus is present in the symbiotic chloroplasts. Physiological and biochemical studies revealed that both chloroplast photosynthetic O₂ evolution and thylakoid photosynthetic electron transport (PET) activity declined after 5 months, attributable to a decline in PSII core activity. Southern blotting with a V. litorea intergenic transcribed spacer (ITS) probe disproved the presence of any whole algal nuclei or nucleomorph in the host sea slug. This suggested the maintenance of algal chloroplast activity may be due to the high autonomy of the chloroplast and/or the interaction between algal chloroplasts and the sea slug nuclear genome. rbcL and rbcS were both located to the ctDNA in V. litorea, where they are co-transcribed and not regulated by light. Surprisingly, these two genes are transcribed only in the early stages of symbiosis and at very low levels in the sea slug. The absence of Rubisco gene transcripts at later stages suggests that active Rubisco in the symbiotic chloroplasts may come from the conserved form of the enzyme stored in the large pyrenoids. rbcL is 1467 bp long and encodes a protein of the "longer C-terminal" type, while rbcS is 430 bp in length and encodes a protein of the "β-purple bacterial like lineage." The high autonomy of chloroplasts, large pyrenoids, and interactions between the algal chloroplasts and sea slug nuclear genome, all contribute to the long-term endosymbiosis.