The chloroplast genome of plants: a unique origin

Abstract

Complete nucleotide sequences of chloroplast genomes are now available for two green plants: Marchantia polymorpha (a liverwort) and Nicotiana tabacum (tobacco). Although these two plant species are taxonomically very distant from one another, their deduced gene organization is remarkably similar. This implies that the chloroplast genomes in all land plants may have arisen from a unique ancestor. Including the unidentified open reading frames, we establish the number of distinct chloroplast genes at about 125, consisting of 4 species of ribosomal RNA genes, 30 or 31 species of transfer RNA genes, and about 90 protein genes. About half of these genes are concerned with the basic mechanisms of gene expression in the chloroplast, such as transcription and translation, and share many features with prokaryotic organisms. Complete sequence analysis revealed that each functional protein complex containing components encoded by the chloroplast genes also contains nuclear-encoded ones, as if some of the genes from each complex have "migrated" to the nucleus from a symbiotic prokaryote during the course of evolution. By this process, the basis of the present nuclear–chloroplast relationships had been established in very early times (300–400 million years ago) before the branching of Bryophytina and Tracheophytina. From this unified single origin the present-day chloroplast genomes in plants have evolved mainly by mutations in nucleotides and rearrangement of DNA, but rarely by changes in gene content.Key words: chloroplast genome, complete DNA sequence, gene organization, Marchantia polymorpha, Nicotiana tabacum, unified origin.