Fig. 1.—

Symbiodinium minutum plastid-associated gene content. (A) A Venn diagram of plastid-associated genes in the plastid genome (green) and the nuclear genome (blue). Gene content of chloroplast genomes of closely related apicomplexans, Chromera velia (Janouskovec et al. 2010) (brown) and Vitrella brassicaformis (purple) (Janouskovec et al. 2010; Obornik et al. 2012), the red alga, Cyanidioschyzon merolae (Ohta et al. 2003) (red), and the green alga, Chlamydomonas reinhardtii (Maul et al. 2002) (dark green) are also shown. (B) Major proteins and protein complexes of the plastid photosynthetic apparatus. Polypeptide subunits of plastid-transferred genes are colored green; nuclear-transferred genes are colored blue, and those not found in the Symbiodinium genome are white. At least in PS I, PS II, and the Cyt b6/f assembly, plastid-encoded subunits are required for initiation of complex assembly, which is a highly ordered process (Nickelsen and Rengstl 2013). PsbE, PsbI, PsbA, and PsbD subunits are first transiently accumulated to build a subcomplex, and then PsbB and PsbC (light green) are incorporated later, for assembly of PS II (Nickelsen and Rengstl 2013). Assembly of PS I is initiated by membrane insertion of PsaA and PsaB, forming a heterodimer, which accounts for almost half the molecular mass of the mature complex (Schottler et al. 2011). In the Cyt b6/f complex, first Cyt b₆ (encoded by petB) and PetD form a subcomplex that serves as a template for assembly of Cyt f (encoded by petA) and PetG (Wollman 1998). (C) psbA minicircle of S. minutum. The core-conserved region contains a high density of IRs (gray) and nine conserved regions (black). Another conserved region shown between C5 and a coding region is a candidate promoter (blue) located just before the TSS. The “CACCAATGCACC” motif (light blue) found in all minicircles containing protein-coding genes is a putative RNA-binding site. A “GAP box” in white represents incomplete sequences that were estimated from PCR products. Compared with red algae (red), the number of plastid-associated genes has been greatly reduced in Symbiodinium. However, in Symbiodinium, the overwhelming majority of these are located in the nucleus (pale blue), rather than in plastids (light green).