A Nucleomorph-Encoded CbbX and the Phylogeny of RuBisCo Regulators

Chloroplasts contain proteins that are encoded by different genetic systems, the plastid genome and the nuclear chromosomes. By comparing the gene content of plastid genomes of different taxa, some predictions about nuclear-encoded genes for plastid proteins are possible. However, early in evolution, many genes were transferred from the plastid to the cell nucleus and are therefore missing from all known plastid genomes and escape such predictions. By sequencing the miniaturized chromosomes of the nucleomorph of the cryptophyte Guillardia theta, as well as the plastid genome, we uncovered two genes encoding CbbX which are predicted to be involved in plastid function. Our findings suggest that (1) red-type plastid rbcLS genes evolved together with cbbX, which is related to cbbX genes of purple bacteria; (2) early in rhodoplast evolution, the cbbX gene was duplicated and transferred into the nucleus; (3) the plastid-encoded LysR transcriptional activator gene, rbcR, is homologous to rbcR and cbbR transcriptional activator genes of purple bacteria and cyanobacteria; and (4) the ancestral plastid probably harbored both types of form I RuBisCo.