Molecular Genetics

Facts

• The RNA polymerases of organelles resemble those of eubacteria more than they do those of eucaryotes. Bacterial and organelle polymerases are sensitive to the same inhibitors and insensitive to inhibitors of eucaryotic RNA polymerases.
• Organelle protein synthesis is sensitive to the same inhibitors that inhibit protein synthesis in eubacteria and insensitive to some that inhibit cytoplasmic protein synthesis. Organelle and bacterial ribosomes are more similar to each other than either are to cytoplasmic ribosomes of eucaryotes.
• Phylogenetic analysis of small subunit rRNA nucleotide sequences suggested that mitochondrial rDNA shared a common ancestor with modern endosymbiotic bacteria (ricketsia, Agrobacterium, Rhizobium).
• Similarly, 16S rDNA phylogenetic analysis suggests that most plastid rDNA genes shared common ancestors with a cyanobacterium. Euglena and Chlamydomonas rDNAs probably arose from a different ancestral cyanobacterial rDNA..
• Intrageneric comparisons of organelle genomes reveal some species with the same gene in both organelle and nuclear genomes. One or the other may be inactive. Occasionally an active gene may be in the organelle for one species and in the nucleus for another of the same family.

Interpretations

• The evolutionary origin of plastids is almost certainly cyanobacterial.
• The evolutionary origin of mitochondria is almost certainly eubacterial.
• Organelles arose from procaryotes that established symbiotic relationships within other cells. This interpretation is known as the endosymbiont theory.
• Endosymbionts must originally have had a larger set of genes, sufficient for autonomous growth. During the course of evolution, many of those genes have been transferred to the nuclear genome or lost.

Further information

• The endosymbiont theory was proposed by Lynn Margulis. (Bibliography)
• An alternate view is that eukaryotes arose by fusion of a eubacterium with an archaebacterium with some of the eubacterial genes winding up in the ancestor of mitochondria (ref).

This is page 1378 of Molecular Genetics by Ulrich Melcher, © 1998, 1999, 2003