Читать книгу Algorithms in Bioinformatics - Paul A. Gagniuc - Страница 45

Both genome and genome-less organelles divide. But how ? Regular bacterial fission (division) uses a dynamin-related protein (DRP) to constrict the membrane at its inner face [121]. However, DRPs are also essential for mitochondrial and peroxisomal fission [122]. Fission is required to provide a population of organelles for daughter cells during mitosis. In contrast to bacterial fission, mitochondria use dynamin and DPRs to constrict the outer membrane (a ring) from the cytosolic face [121]. For instance, the unicellular eukaryote Trichomonas vaginalis is a parasite that uses hydrogenosomes instead of regular mitochondria. The role of DRPs in the division of the hydrogenosome is similar to that described for peroxisomes and mitochondria [123]. Moreover, plastids use similar mechanisms and a plant-specific DPR [124, 125]. In eukaryotes, dynamin and DPRs have their genes stored in the nuclear genome. Thus, control over the division of membrane-bound organelles is held by the nuclear genome. Genes encoding DPRs were once present in the symbiotic α-proteobacterium ancestors of mitochondria or in the symbiotic cyanobacterium ancestors of the plastids. In other words, the alternative self-assembly of a complementary dynamin constriction mechanism on the outer membrane, allowed a transition of the organellar fission genes to the nuclear genome for synchronization of cell division. Moreover, this complementary mechanism allowed a reductive evolution up to the point of complete genome elimination in some organelles, such as in the case of hydrogenosomes. But how do organelle genes physically get into the nucleus to recombine with chromosomal DNA? Insertion of DNA from organelle genomes into the nucleus is DNA-mediated (RNA-mediated insertions may also occur) through a process called HGT [126]. As a side note, peroxisomes are single-membrane organelles that catalyze the breakdown of very-long-chain fatty acids through beta-oxidation [127]. Peroxisomes are a hybrid of mitochondrial and ER-derived (ER – endoplasmic reticulum) pre-peroxisomes [128].