Redox signalling in chloroplasts and mitochondria: genomic and biochemicalevidence for two-component regulatory systems in bioenergetic organelles

Redox chemistry is central to the primary functions of chloroplasts and mit ochondria, that is, to energy conversion in photosynthesis and respiration. However, these bioenergetic organelles always contain very small, speciali zed genetic systems, relics of their bacterial origin. At huge cost, organe llar genomes contain, typically, a mere 0.1% of the genetic information in a eukaryotic cell. There is evidence that chloroplast and mitochondrial gen omes encode proteins whose function and biogenesis are particularly tightly governed by electron transfer. We have identified nuclear genes for 'bacte rial' histidine sensor kinases and aspartate response regulators that seem to be targeted to chloroplast and mitochondrial membranes. Sequence similar ities to cyanobacterial redox signalling components indicate homology and s uggest conserved sensory and signalling functions. Two-component redox sign alling pathways might be ancient, conserved mechanisms that permit endogeno us control over the biogenesis, in situ, of bioenergetic complexes of chlor oplasts and mitochondria.