Chloroplasts Contain Large DNAs Often Encoding More Than a Hundred Proteins

Like mitochondria, chloroplasts are thought to have evolved from an ancestral endosymbiotic photosynthetic bacterium (see Figure 12-7). However, the endosymbiotic event that gave rise to chloroplasts occurred more recently (1.2 billion–1.5 billion years ago) than the event that led to the evolution of mitochondria (1.5 billion–2.2 billion years ago). Consequently, contemporary chloroplast DNAs show less structural diversity than do mtDNAs. Also like mitochondria, chloroplasts contain multiple copies of the organelle DNA as well as ribosomes, which synthesize some chloroplast DNA–encoded proteins using the standard genetic code. Like plant mtDNA, chloroplast DNA is inherited exclusively in a uniparental fashion through the female parent (egg). Other chloroplast proteins are encoded by nuclear genes, synthesized on cytosolic ribosomes, and then incorporated into the organelle (see Chapter 13).

In higher plants, chloroplast DNA molecules are 120–160 kb long, depending on the species. Plant chloroplast DNAs are long head-to-tail linear concatemers plus recombination intermediates between these long linear molecules. They contain 120–135 genes, 130 in the important model plant Arabidopsis thaliana. A. thaliana chloroplast DNA encodes 76 protein-coding genes and 54 genes with RNA products such as rRNAs and tRNAs. Chloroplast DNAs encode the subunits of a bacteria-like RNA polymerase, and they express many of their genes from polycistronic operons, as in bacteria (see Figure 5-13a). Some chloroplast genes contain introns, but these introns are similar to the specialized introns found in some bacterial genes and in mitochondrial genes from fungi and protozoans, rather than the introns of nuclear genes. Many genes essential for chloroplast function have been transferred to the nuclear genome of plants over evolutionary time. Recent estimates from sequence analysis of the A. thaliana and cyanobacterial genomes indicate that somewhat less than 4500 genes have been transferred from the original endosymbiont to the nuclear genome.

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Methods similar to those used for the transformation of yeast cells (see Chapter 6) have been developed for stably introducing foreign DNA into the chloroplasts of higher plants. The large number of chloroplast DNA molecules per cell permits the introduction of thousands of copies of an engineered gene into each cell, resulting in extraordinarily high levels of foreign protein production, comparable with that achieved with engineered bacteria. Chloroplast transformation has led to the engineering of plants that are resistant to bacterial and fungal infections, drought, and herbicides as well as to plants that can be used to make human pharmaceutical drugs (called pharming). The first such pharming drug, approved in the United States for use in adults in 2012 and children in 2014, is an enzyme to treat Gaucher’s disease, a genetic disorder. This approach might also be used for the engineering of food crops containing high levels of all the amino acids essential to humans.