In the July 2000.issue of Trends in Genetics (16(7):315-20, Dr. Michael Blanchard of the National Center for Genome Research, Santa Fe, NM, and Professor M. Lynch of Biology Department, University of Oregon, have reviewed the present status of knowledge on the mechanisms of integration of organellar genes into nuclear genome. During the course of evolution extending over millions of years, many of the chloroplast and mitochondrial genes, that produce essential proteins, migrated and became integrated into the nuclear genome. In order for the migrated gene to be expressed and function, it acquires a transit peptide to enter the organelle. Once the proteins are inside the organelle, they are properly folded, modified and in some cases shaped into larger protein complexes to become as efficient as organellar genes to perform specific functions. One of the postulates explaining the replacement of an organellar gene with a nuclear one is that the nuclear DNA is linear compared to circular organellar genome, and the linear arrangement of DNA is an advantage over the circular one, which characterizes mitochondria and chloroplasts.
Recent studies have further shown that the movement of copies of organellar DNA to the nucleus, marks the beginning of the process of mitochondrial/chloroplast gene transfer. Yeast mutants provide good evidence that organellar DNA breaks up into fragments and the latter escape at leaky sites of the organelle or through its membrane transporters. Thereafter, these fragments ligate with other DNA/RNA and enter the nucleus and become integrated into the nuclear genome.
After the migration, both organellar and integrated genes function, contributing to an excess of enzyme activity in the form of enhanced protein quantity. With accumulation of deleterious mutations in the organellar gene, the original gene’s activity declines till it can no longer function. On the other side of the scenario, beneficial mutations occur in the nuclear copy at a frequency higher than in the original copy, until the efficiency of the migrated gene reaches 100% and that of the original gene declining to zero. It is at this point there may be a loss of the original organellar gene; because of its loss of function, such an event is of no consequence to the cell.
Gene transfer also takes place in the opposite direction as demonstrated by the recent sequencing studies on mitochondrial genomes of Arabidopsis. These studies have shown that in addition to organellar genes, the mitochondrial genome also contains fragments of nuclear genes, plastid DNA, retrotransposons and sequences similar to plant pathogen RNA viruses.