Chloroplast Genome

Abstract

Chloroplasts contain a genome that is a relic of the endosymbiont that gave rise to the organelle. These genomes typically contain 100–200 genes and encode proteins important for photosynthesis and other chloroplast functions, although dinoflagellate algae have a greatly reduced and fragmented chloroplast genome. Many, but not all, nonphotosynthetic taxa retain a remnant chloroplast genome. In some organisms the chloroplast genome may be retained to allow redox‐mediated control of gene expression, whereas in others it may continue to exist because transfer of essential genes to the nucleus is no longer possible. Multiple ribonucleic acid (RNA) polymerases are involved in chloroplast transcription in land plants, and post‐transcriptional processing involving nuclear‐encoded RNA‐binding proteins also plays an important part in the expression of the chloroplast genome. Gene expression may be regulated at a number of levels, and redox poise in the organelle may be particularly important for this. Key Concepts Chloroplasts originated in the ancestor of plants and red and green algae by endosymbiotic acquisition of a cyanobacterium, and then spread to many other eukaryotic lineages. There are other examples of stable acquisition of a cyanobacterium by a nonphotosynthetic host. Chloroplast genomes are typically 100–200 kbp in size, and include a set of genes for proteins essential to photosynthesis. Other genes present in the ancestral symbiont have been lost or relocated to the nucleus. Many nonphotosynthetic organisms retain remnant chloroplasts, with genomes, reflecting the fact that photosynthesis is not the only biochemical process that takes place in the chloroplast. In many organisms, gene transfer from chloroplast to nucleus can still take place, at an unexpectedly high frequency. Establishment of the chloroplast has resulted in the development of nuclear‐encoded RNA‐binding protein families and, in land plants, additional RNA polymerases that play a central role in chloroplast gene expression. Post‐transcriptional RNA processing plays an important role in chloroplast gene expression. In photosynthetic organisms, chloroplast gene expression is closely linked to the organelle's redox poise. The chloroplast can also influence the expression of nuclear genes.