Plastid Genomes

Plastids possess their own genome, the plastome, and a specific machinery to decode its genetic information. The first evidence for the presence of heritable material in plastids was reported at the beginning of the last century and was based on observations of non-Mendelian inheritance of variegated leaf phenotypes. More than 50 years later, specific plastid-localized DNA was identified. The first complete plastid genome sequences were published in 1986. Since then, more than 30 genomes from embryophytes and eukaryotic algae have been deciphered. The typical plastid genome consists of multiple copies of a basic unit of double-stranded DNA of species-specific length. Circular and linear molecules in monomeric and multimeric forms have been observed. Generally, several plastid DNA molecules are organized into nucleo/protein complexes, so-called nucleoids. Whereas the physical organization of plastid genomes of embryophytes is highly conserved, algal plastid genomes appear highly divergent. The plastid genome of embryophytes typically consists of units of 120 to 160 kbp in length, with each unit generally subdivided in four sections with two of the sections made up of two identical copies of a large inverted repeat region. In contrast, plastid genome units of algae show large size variations from less than 100 kbp to more than 1.5 Mbp and, instead of a single genome unit, certain dinoflagellates contain several 2 to 3 kbp minicircles mostly encoding only a single gene. Moreover, in some protozoan parasites, relic non-photosynthetic plastid-like organelles of algal origin, so-called apicoplasts, contain small 35 kbp genomes. The evolutionary origin of plastid genomes traces back to the engulfment of a free-living cyanobacterium by a eukaryotic host more than 1.2 billion years ago. The functional and genetic integration of the former autarkic cyanobacterium into the newly emerging photosynthetic eukaryotic cell was accompanied by an intermixing and restructuring of genomes. During the course of this process the original cyanobacterial genome, encoding several thousand genes, was intensely reduced such that the present-day plastid genome contains only 100 to 250 genes. A large proportion of the original genes have been translocated to the nucleus and most gene products for plastid functions have to be reimported into the organelle. Gene transfer from the plastid genome to the mitochondrial genome also took place. However, no evidence for retrograde gene transfer from the nuclear or mitochondrial genome to the plastid genome has been found. Since DNA transfer from plastids to the nuclear genome is still an ongoing process, the question arises why it was not driven to completion. Most of the genes remaining in the plastid genome encode either components of the photosynthetic apparatus or of the transcription/translation apparatus of the organelle. Probably a core set of plastid genes must be maintained because of their regulatory properties. In this respect, it has been hypothesized that to prevent the production of harmful oxygen radicals, it is favourable to maintain certain redoxregulated genes inside the plastid compartment.