1. LSTM TA 10/J Laboratoire des Symbioses Tropicales et Méditerranéennes, UMR 113, IRD/CIRAD/INRA/ENSA-M. Campus de Baillarguet, 34398 Montpellier Cedex 5, France
2. CEA/Cadarache DEVM-Laboratoire d'Écophysiologie de la Photosynthèse, UMR 163-CNRS-CEA, 13108 Saint Paul lez Durance Cedex, France
3. CEA/Cadarache DEVM-Laboratoire de Bioénergétique Cellulaire, UMR 163-CNRS-CEA, Univ-Méditerranée CEA1000, 13108 Saint Paul lez Durance Cedex, France

Correspondence to: André Verméglio³ Correspondence and requests for materials should be addressed to A.V. (e-mail: Email: avermeglio@cea.fr). GenBank accession codes for the (bacterio)phytochrome sequences are AF182374 (Bradyrhizobium ORS278), AB00139 (Synechocystis PCC6803), AAF12261 (D. radiodurans), AF064527 (R. centenum), X17342 (Arabidopsis thaliana). The genomic organization of Rps. palustris was deduced from the genome database at http://spider.jgi-psf.org/JGI_microbial/html/index.html.

Abstract

Plants use a set of light sensors to control their growth and development in response to changes in ambient light. In particular, phytochromes exert their regulatory activity by switching between a biologically inactive red-light-absorbing form (Pr) and an active far-red-light absorbing form (Pfr)^{1, 2}. Recently, biochemical and genetic studies have demonstrated the occurrence of phytochrome-like proteins in photosynthetic and non-photosynthetic bacteria^{3, 4, 5, 6, 7}—but little is known about their functions. Here we report the discovery of a bacteriophytochrome located downstream from the photosynthesis gene cluster in a Bradyrhizobium strain symbiont of Aeschynomene. The synthesis of the complete photosynthetic apparatus is totally under the control of this bacteriophytochrome. A similar behaviour is observed for the closely related species Rhodopseudomonas palustris, but not for the more distant anoxygenic photosynthetic bacteria of the genus Rhodobacter, Rubrivivax or Rhodospirillum. Unlike other (bacterio)phytochromes, the carboxy-terminal domain of this bacteriophytochrome contains no histidine kinase features. This suggests a light signalling pathway involving direct protein–protein interaction with no phosphorelay cascade. This specific mechanism of regulation may represent an important ecological adaptation to optimize the plant–bacteria interaction.