Form II Rubisco
1, 5-bisphosphate carboxylase/oxygenase (RUBISCO)
Rubisco is the key enzyme catalyzing CO2 fixation and is thus essential for photosynthetic organisms. Most algae possess a Form I Rubisco (rbcI) in the plastid that is composed of eight identical large subunits and eight identical small subunits. In chlorophytes, the large subunits are encoded in the plastid genome while the small subunits in the nucleus (due to lateral gene transfer). In most non-green lineages, genes for both subunits are located in the plastid. Phylogenetic studies of Rubisco large subunit reveal that these genes were acquired from the cyanobacterial progenitor of plastids in green algae and from proteobacteria in the non-green plastids (Delwiche and Palmer 1996, Paoli et al 1998, Uchino and Yokota 2003).
Photosynthetic dinoflagellates are an exception with respect to Rubisco as well as many other features. While some species possess “aberrant” plastids that are considered “transient” symbionts acquired more recently from other eukaryotic algae (Schnepf and Elbrachter 1999, Tengs et al. 2000), the majority of the approximately 1000 photoautotrophic species have plastids with distinct characteristics, and their origin still is under debate (Ishida and Green 2002, Yoon et al. 2002). These “genuine” dinoflagellate plastids are surrounded by three membranes, and contain peridinin as the major carotenoid pigment; their genomes are composed of single-gene minicircles and are devoid of rbcI (Zhang and Green 1999, Barbrook and Howe 2000). The plastids instead contain a nucleus-encoded Rubisco (Morse et al. 1995, Whitney et al. 1995, Rowan et al. 1996), consisting of a single subunit equivalent to the large subunit of form I Rubisco, otherwise found only in bacteria (Nargang et al. 1984, Whitney et al. 1995, Whitney and Andrew 1998, Tabita 1999). Due to the high similarity to bacterial form II Rubisco gene (rbcII), dinoflagellate Rubisco genes are thought to have arisen from bacteria through lateral gene transfer (Morse et al. 1995).
Only three dinoflagellate species have been studied so far with respect to rbcII: first was the discovery of dinoflagellate rbcII in Lingulodinium polyedrum (formerly Gonyaulax polyedra) (Morse et al. 1995) then the characterization of dinoflagellate rbcII in the zooxanthalla Symbiodinium sp. (Rowan et al. 1996). In Prorocentrum minimum (Parvillard) Schiller, Rubisco gene occurs as numerous TUs each containing multiple CUs (Fig. 1).
Using Real-Time PCR, we estimated that there were a total of 148 ± 16 copies of this gene in the genome (Fig. 2)
These numerous copies of rbcL are transcribed in four tandem repeats (TU), which appear to be translated to tetrameric polyproteins (Fig. 3). Apparently, the polyprotein is cleaved to a short fragment containing a Rubisco with a signal peptide, which is imported to the chloroplast before being cleaved again to yield a mature Rubisco (Fig. 4).