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deletions of mtDNA might even be at a selective advantage in replication because they can replicate faster.

All cells have mitochondria, yet mutations in mtDNA affect only some tissues. Those most usually affected are tissues that have a high requirement for ATP produced by oxidative phosphorylation and tissues that require most of or all the mtDNA in the cell to synthesize sufficient amounts of functional mitochondrial proteins. Leber's hereditary optic neuropathy (degeneration of the optic nerve, accompanied by increasing blindness), for instance, is caused by a missense mutation in the mtDNA gene encoding subunit 4 of the NADH-CoQ reduc-tase. Any of several large deletions in mtDNA causes another set of diseases including chronic progressive external ophthalmoplegia and Kearns-Sayre syndrome, which are characterized by eye defects and, in Kearns-Sayre syndrome, also by abnormal heartbeat and central nervous system degeneration. A third condition, causing "ragged" muscle fibers (with improperly assembled mitochondria) and associated uncontrolled jerky movements, is due to a single mutation in the T^CG loop of the mitochondrial lysine tRNA. As a result of this mutation, the translation of several mito-chondrial proteins apparently is inhibited I

Chloroplasts Contain Large Circular DNAs Encoding More Than a Hundred Proteins

As we discuss in Chapter 8, the structure of chloro-plasts is similar in many respects to that of mitochondria. Like mitochondria, chloroplasts contain multiple copies of the organellar DNA and ribosomes, which synthesize some chloroplast-encoded proteins using the "standard" genetic code. Other chloroplast proteins are fabricated on cytosolic ribosomes and are incorporated into the organelle after translation (Chapter 16).

Chloroplast DNAs are circular molecules of 120,000160,000 bp, depending on the species. The complete sequences of several chloroplast DNAs have been determined. Of the «120 genes in chloroplast DNA, about 60 are involved in RNA transcription and translation, including genes for rRNAs, tRNAs, RNA polymerase subunits, and riboso-mal proteins. About 20 genes encode subunits of the chloro-plast photosynthetic electron-transport complexes and the F0F1 ATPase complex. Also encoded in the chloroplast genome is the larger of the two subunits of ribulose 1,5-bisphosphate carboxylase, which is involved in the fixation of carbon dioxide during photosynthesis.

Reflecting the endosymbiotic origin of chloroplasts, some regions of chloroplast DNA are strikingly similar to the DNA of present-day bacteria. For instance, chloroplast DNA encodes four subunits of RNA polymerase that are highly homologous to the subunits of E. coli RNA poly-merase. One segment of chloroplast DNA encodes eight proteins that are homologous to eight E. coli ribosomal proteins; moreover, the order of these genes is the same in the two DNAs.

Although the overall organization of chloroplast DNAs from different species is similar, some differences in gene composition occur. For instance, liverwort chloroplast DNA has some genes that are not detected in the larger tobacco chloroplast DNA, and vice versa. Since chloroplasts in both species contain virtually the same set of proteins, these data suggest that some genes are present in the chloroplast DNA of one species and in the nuclear DNA of the other, indicating that some exchange of genes between chloroplast and nucleus occurred during evolution. I

Methods similar to those used for the transformation of yeast cells (Chapter 9) have been developed for stably introducing foreign DNA into the chloroplasts of higher plants. The large number of chloro-plast 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. Chloroplast transformation has recently led to the engineering of plants that are resistant to bacterial and fungal infections, drought, and herbicides. The level of production of foreign proteins is comparable with that achieved with engineered bacteria, making it likely that chloroplast transformation will be used for the production of human pharmaceuticals and possibly for the engineering of food crops containing high levels of all the amino acids essential to humans. I

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