Molecular phylogeny and evolution of the plastid and nuclear encoded cbbX genes in the unicellular red alga Cyanidioschyzon merolae

The cbbX gene is generally encoded in proteobacterial genomes and red-algal plastid genomes. In this study, we found two distinct cbbX genes of Cyanidioschyzon merolae, a unicellular red alga, one encoded in the plastid genome and the other encoded in the cell nucleus. The phylogenetic tree inferred from cbbX genes and strongly conserved gene organization (rbcLS-cbbX) suggests that the plastid-encoded cbbX gene of C. merolae came from an ancestral proteobacterium by horizontal gene transfer. On the other hand, the nuclear-encoded cbbX gene of C. merolae was classified in another cluster together with the nucleomorph-encoded cbbX gene of Guillardia theta. Furthermore, expression of the two cbbX genes were regulated differently in response to extracellular CO2 concentration. Our results imply that cbbX gene in the plastid genome was copied and transferred to the cell nucleus after horizontal gene transfer of RuBisCo operon from ancestral beta-proteobacteria at comparatively early stage, and that each cbbX evolved in different ways.