Interactions between the nucleus and organelles of cytoplasm have a major role in eukaryotic cellular metabolism. ATPases, some cytochromes, for example, the yeast cytochrome c oxidase complex, are assembled posttranslationally in mitochondria and chloroplasts from some polypeptide subunits synthesized in the nucleus, others in the organelle. The genetic materials of the nucleus, mitochondrion, and chloroplast are usually maintained discretely in each organelle without duplication elsewhere. Exceptions are known as promiscuous DNA, defined as a nucleotide sequence that occurs in more than one of these three membrane-bound organelles. This should be distinguished from the DNA of promiscuous plasmids, which can be transferred to a wide range of prokaryotic cells.
Promiscuous DNA was first reported upon the discovery that the genomes of mitochondria and chloroplasts of maize have a 12-kb DNA sequence in common. The most attractive conclusion is that the chloroplast DNA became incorporated in the mitochondrial genome after gene duplication and transposition (1).
A similar phenomenon was found in Saccharomyces cerevisiae. A piece of DNA is common to the mitochondrial and nuclear genomes. There is good evidence that the "jumping" occurred in the direction nucleus ^ mitochondrion (2).
A third example comes from the sea urchin, Strongylocentrotus purpuratus, whose sections of mitochondrial DNA, coding for cytochrome oxidase subunit 1 and the 3′ 16 S RNA of ribosomes, are found in the nuclear genome. This is probably a result of a chromosomal transposition about 25 million years ago, followed by chromosomal rearrangements and single nucleotide substitutions (3).
