Nucleotide sequence of the plastid genome of an unicellular red alga Cyanidioschyzon merolae, which is thought to be among the most primitive plants, have been completely sequenced (Ohta et al. 2003). Its genes, the gene organization and the gene content have been analysed. The size of the plastid genome of C. merolae is approximately 150 kbp, and this size is almost as large as those of other plastid genomes. Spacer regions between genes are generally very short and many of the genes overlap each other. No introns have been found in this plastid genome. The number of genes on this plastid genome is approximately 250, and this value is 1.7 times as many as those of the plastid genomes of chlorophytes (land plants and algae). The plastid genome of C. merolae, though, lacks inverted repeat sequences that are typical in plastid genomes and cyanobacteria. The feature of the plastid genome of C. merolae is its gene content. It contains several genes that are involved in the biosynthesis in the plastid. The most striking is the existence of genes concerning to the synthesis of lipid A which is the material of the bacterial peptidglycan layer. This must be a strong evidence of the endosymbiosis that an ancestral eukaryotic cell engulfed an ancestral cyanobacterium, and the cyanobacterium evolved into the plastid.
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