1 Research Department of Genetics, Evolution and Environment, Darwin Building, University College London, Gower Street, London WC1E 6BT, UK. Electronic address: j.f.allen@ucl.ac.uk.
2 Institute of Molecular Evolution, Heinrich-Heine-University, 40225 Düsseldorf, Germany.
Genes in mitochondria and chloroplasts are co-located with their gene products to permit regulation of trans-membrane electron transport at the energetic boundary of the cell.