Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 18

Essentially all eukaryotic cells contain something, the mitochondria, that ties them to the world of bacteria. The mitochondria of eukaryotic cells are the sites of efficient adenosine triphosphate (ATP) generation through respiration. Evidence, including the sequences of many genes in their rudimentary chromosomes, indicates that the mitochondria of eukaryotes are descended from free-living bacteria from the Alphaproteobacteria that formed a symbiosis with a primitive ancestor of eukaryotes. A specialized type of symbiosis, where the symbiont resides entirely within another organism, is called endosymbiosis.

Plant cells and some unicellular eukaryotic cells also contain chloroplasts, the site of photosynthesis. Like mitochondria, chloroplasts are also descended from free-living bacteria, in this case Cyanobacteria. Mitochondria and chloroplasts resemble bacteria in many ways. For instance, they contain DNA that encodes the components of oxidative phosphorylation and photosynthesis, as well as rRNAs and transfer RNAs (tRNAs). Even more striking, the mitochondrial and chloroplast rRNA and ribosomal proteins, as well as the membranes of the organelles, more closely resemble those of bacteria than they do those of eukaryotes. Comparisons of the sequences of highly conserved organelle genes, like the rRNAs, with those of bacteria also support this view of chloroplasts and mitochondria (see Yang et al., Suggested Reading).

Mitochondria and chloroplasts may have come to be associated with early eukaryotic/archaeal cells when these cells engulfed bacteria to take advantage of their superior energy-generating systems or their ability to obtain energy from light through photosynthesis. The engulfed bacteria eventually lost many of their own genes, which moved to the chromosome, from where they are expressed and their products are transported back into the organelle. The organelles had by then lost their autonomy and had become permanent endosymbionts of the eukaryotic cells. In one view, the role of endosymbiosis in the evolution of eukaryotes calls into quest ion the use of phylogenetic trees as a tool to describe the interrelationship of entire organisms, given that many organisms represent a conglomeration of genomes (see Koonin, Suggested Reading).

Interestingly, members of a large newly identified group within bacteria with no cultured representatives, called the Candidate Phyla Radiation (Figure 1), have relatively small genomes and, to varying degrees, limited metabolic capacities. This has led to the idea that many of these lineages may be symbionts, something that has been shown in some cases.