The fact that plastids contain their own DNA has been known since the 1960s and has been a central tenet in the construction of theories about plastid evolution and endosymbiosis. The genome of the plastid in higher plants contains approximately 120–160 genes, the products of which function in the plastid in a variety of ways, most importantly in the process of photosynthesis, but also in the translation machinery of the plastid itself. All of the proteins which are encoded by the plastid genome function within the plastid and are not exported. Thus the plastid genome is a distinct ‘in-house’ genome, which is replicated and transcribed to produce proteins crucial for plastid function. Many of the genes that were present on the genome of the original endosymbiont have migrated into the nuclear genome, leaving behind a residual number of genes which constitute the modern plastid genome that is found today in higher plants. In this chapter we consider how the plastid genome is structured and stored in the plastid, the sequence of the plastid genome and what it encodes and how mRNA molecules resulting from transcription of the plastid genome are translated within the plastid itself.
The architecture of the plastid genome
The DNA in the plastid (ptDNA) has long been considered to exist as a closed, circular molecule of double-stranded DNA of between 120 and 160 kb in size. The evidence for such a circular structure came largely from scanning electron micrographs of isolated ptDNA, which show distinct circular structures, along with shorter linear DNA molecules (Fig. 3.1).