ABSTRACT
The chloroplasts of most dinoflagellates are unusual in that they are surrounded by three membranes and contain the unique carotenoid peridinin as the principal xanthophyll. The ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) of dinoflagellates in dinoflagellate chloroplasts was found here to also be unusual. Unlike other eukaryotes, dinoflagellates containing peridinin use a form of rubisco (form II) previously found only in some species of proteobacteria. Furthermore, this rubisco is not encoded in the chloroplast DNA, as is the case in other organisms, but is encoded by the the nuclear DNA. The unusual nature of this enzyme and location of its gene support the idea that dinoflagellate chloroplasts may have had a distinctive evolutionary origin.