evidence for two-component regulatory systems in bioenergetic organelles

Redox chemistry is central to the primary func-tions of chloroplasts and mitochondria, that is, to energy conversion in photosynthesis and respir-ation. However, these bioenergetic organelles always contain very small, specialized genetic systems, relics of their bacterial origin. At huge cost, organellar genomes contain, typically, a mere 0.1 % of the genetic information in a eukaryotic cell. There is evidence that chloroplast and mito-chondrial genomes encode proteins whose func-tion and biogenesis are particularly tightly governed by electron transfer. We have identified nuclear genes for ‘bacterial ’ histidine sensor kin-ases and aspartate response regulators that seem to be targeted to chloroplast and mitochondrial membranes. Sequence similarities to cyano-bacterial redox signalling components indicate homology and suggest conserved sensory and signalling functions. Two-component redox sig-nalling pathways might be ancient, conserved mechanisms that permit endogenous control over the biogenesis, in situ, of bioenergetic complexes of chloroplasts and mitochondria. Key words: evolution, photosynthesis, redox, regulation, respira-tion. Abbreviations used: FNR, fumarate and nitrate reductase. 1To whom correspondence should be addressed (e-mail john.allen!plantbio.lu.se)