CHLOROPLAST GENES ARE EXPRESSED DURING INTRACELLULAR SYMBIOTIC ASSOCIATION OF VAUCHERIA-LITOREA PLASTIDS WITH THE SEA SLUG ELYSIA-CHLOROTICA

The marine slug Elysia chlorotica (Gould) forms an intracellular symbi osis with photosynthetically active chloroplasts from the chromophytic alga Vaucheria litorea (C. Agardh). This symbiotic association was ch aracterized over a period of 8 months during which E. chlorotica was d eprived of V. litorea but provided with light and CO2. The fine struct ure of the symbiotic chloroplasts remained intact in E. chlorotica eve n after 8 months of starvation as revealed by electron microscopy, Sou thern blot analysis of total DNA from E. chlorotica indicated that alg al genes, i.e., rbcL, rbcS, psaB, psbA, and 16S rRNA are present in th e animal. These genes are typically localized to the plastid genome in higher plants acid algae except rbcS, which is nuclear-encoded in hig her plants and green (chlorophyll a/b) algae. Our analysis suggests, h owever, that similar to the few other chromophytes (chlorophyll a/c) e xamined, rbcS is chloroplast encoded in V. litorea. Levels of psbA tra nscripts 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 rR NA transcripts remained constant throughout the starvation period. The levels of the photosystem II proteins, D1 and CP43, were high at 2 an d 4 months of starvation and remained constant at a lower steady-state level after 6 months. In contrast, D2 protein levels, although high a t 2 and 4 months. were very low at all other periods of starvation. At 8 months, de novo synthesis of several thylakoid membrane-enriched pr oteins, including D1, still occurred. To our knowledge, these results represent the first molecular evidence for active transcription and tr anslation of algal chloroplast genes in an animal host and are discuss ed in relation to the endosymbiotic theory of eukaryote origins.