John F. Allen
Research Department of Genetics, Evolution and Environment
University College London
"Why have chloroplasts and mitochondria retained genomes?"
Chloroplasts and mitochondria are subcellular bioenergetic organelles with their own genomes and genetic systems. DNA replication and its transmission to daughter organelles produces cytoplasmic inheritance of characters associated with primary events in photosynthesis and respiration. The prokaryotic ancestors of chloroplasts and mitochondria were endosymbionts whose genes became copied to the genomes of their cellular hosts. These copies are now nuclear chromosomal genes that encode either cytosolic proteins or precursor proteins imported into the organelle into which the endosymbiont evolved. What accounts for the retention of genes for the complete synthesis within chloroplasts and mitochondria of only a tiny minority of their protein subunits? One hypothesis is that expression of genes for components of electron transport chains must respond to physical environmental change by means of a direct and unconditional regulatory control—control exerted by change in the redox state of the corresponding gene product (1). This hypothesis proposes that, to preserve function, an entire redox regulatory system must be retained within its original membrane-bound compartment together with the DNA upon which it acts. Co-location of gene and gene product for Redox Regulation of gene expression (CoRR) is an hypothesis in agreement with the results of a variety of experiments designed to test it and that seem to have no other satisfactory explanation (2). I present evidence relating to the CoRR hypothesis, and consider the mechanism by which transcription of reaction centre genes is coupled to photosynthetic electron transport in chloroplasts.
- Allen JF (2017) J. Theor. Biol. 434: 50-57.
- Allen JF (2015) PNAS 112: 10231–10238.