The nature of the organelle genes located in the nucleus of the cell demonstrates that an extensive transfer of genes from organelle to nuclear DNA has occurred in the course of eukaryotic evolution. Such successful gene transfer is expected to be rare, because any gene moved from the organelle needs to adapt to both nuclear transcription and cytoplasmic translation requirements. In addition, the protein needs to acquire a signal sequence that directs it to the correct organelle after its synthesis in the cytosol. By comparing the genes in the mitochondria from different organisms, we can infer that some of the gene transfers to the nucleus occurred relatively recently. The smallest and presumably most highly evolved mitochondrial genomes, for example, encode only a few hydrophobic inner-membrane proteins of the electron-transport chain, plus ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs). Other mitochondrial genomes that have remained more complex tend to contain this same subset of genes along with others (Figure 14–60). The most complex mitochondrial genomes include genes that encode components of the mitochondrial genetic system, such as RNA polymerase subunits and ribosomal proteins; these same genes are found in the cell nucleus in yeast and all animal cells.
The proteins that are encoded by genes in the organellar DNA are synthesized on ribosomes within the organelle, using organelle-produced messenger RNA (mRNA) to specify their amino acid sequence (Figure 14–61). The protein traffic between the cytosol and these organelles seems to be unidirectional: proteins are normally not exported from mitochondria or chloroplasts to the cytosol. An important exception occurs when a cell is about to undergo apoptosis. As will be
Figure 14–60 Comparison of mitochondrial genomes. Less complex mitochondrial genomes encode subsets of the proteins and ribosomal rNas that are encoded by larger mitochondrial genomes. In this comparison, there are only five genes that are shared by the six mitochondrial genomes; these encode ribosomal rNas (rns and rnl), cytochrome b (cob), and two cytochrome oxidase subunits (cox1 and cox3). Blue indicates ribosomal rNas; green, ribosomal proteins; and brown, components of the respiratory chain and other proteins. (adapted from
M.W. Gray, G. Burger and B.F. Lang, Science 283:1476–1481, 1999.) discussed in detail in Chapter 18, during apoptosis the mitochondrion releases proteins (most notably cytochrome c) from the crista space through its outer mitochondrial membrane, as part of an elaborate signaling pathway that is triggered to cause cells to undergo programmed cell death.
the Fission and Fusion of Mitochondria are topologically Complex processes
In mammalian cells, mitochondrial DNA makes up less than 1% of the total cellular DNA. In other cells, however, such as the leaves of higher plants or the very large egg cells of amphibians, a much larger fraction of the cellular DNA may be present in mitochondria or chloroplasts (Table 14–2), and a large fraction of the total RNA and protein synthesis takes place in the organelles.
Mitochondria and chloroplasts are large enough to be visible by light microscopy in living cells. For example, mitochondria can be visualized by expressing in cells a genetically engineered fusion of a mitochondrial protein linked to green