A New Type of Bacteriophytochrome Acts in Tandem with a Classical Bacteriophytochrome to Control the Antennae Synthesis in Rhodopseudomonas palustris

Eric Giraud,Sébastien Zappa,Laurie Vuillet,Jean-Marc Adriano,Laure Hannibal,Joël Fardoux,Catherine Berthomieu,Pierre Bouyer,David Pignol,André Verméglio

doi:10.1074/jbc.m506890200

Abstract

Phytochromes are chromoproteins found in plants and bacteria that switch between two photointerconvertible forms via the photoisomerization of their chromophore. These two forms, Pr and Pfr, absorb red and far-red light, respectively. We have characterized the biophysical and biochemical properties of two bacteriophytochromes, RpBphP2 and RpBphP3, from the photosynthetic bacterium Rhodopseudomonas palustris. Their genes are contiguous and localized near the pucBAd genes encoding the polypeptides of the light harvesting complexes LH4, whose synthesis depends on the light intensity. At variance with all (bacterio)phytochromes studied so far, the light-induced isomerization of the chromophore of RpBphP3 converts the Pr form to a form absorbing at shorter wavelength around 645 nm, designated as Pnr for near red. The quantum yield for the transformation of Pr into Pnr is about 6-fold smaller than for the reverse reaction. Both RpBphP2 and RpBphP3 autophosphorylate in their dark-adapted Pr forms and transfer their phosphate to a common response regulator Rpa3017. Under semiaerobic conditions, LH4 complexes replace specifically the LH2 complexes in wild-type cells illuminated by wavelengths comprised between 680 and 730 nm. In contrast, mutants deleted in each of these two bacteriophytochromes display no variation in the composition of their light harvesting complexes whatever the light intensity. From both the peculiar properties of these bacteriophytochromes and the phenotypes of their deletion mutants, we propose that they operate in tandem to control the synthesis of LH4 complexes by measuring the relative intensities of 645 and 710 nm lights.

Highlights

What is the significance of the tandem organization of the two bacteriophytochromes genes? Do these two bacteriophytochromes possess different properties? Are these two bacteriophytochromes involved in the regulation of the closely located pucBAd genes or in different regulatory processes? In this report, we show by combining genetics, biochemical and biophysical approaches that the bacteriophytochrome RpBphP3 possesses unusual photochemical properties and works in tandem with RpBphP2 to regulate the synthesis of the LH4 complexes
To prove that RpBphP2 and RpBphP3 encode functional bacteriophytochromes, they were co-expressed with the heme oxygenase gene in E. coli and subsequently purified
Based on the results described above, we conclude that the lightinduced dephosphorylation of RpBphP2 and RpBphP3 is the first step of the signal transduction pathway of the LH4 synthesis

Summary

Introduction

Some exceptions occur such as the bacteriophytochromes of the photosynthetic bacteria Bradyrhizobium (BrBphP) and Rps. palustris (denoted here RpBphP1), which do not possess a histidine kinase motif but an S-box domain that might be involved in protein-protein interactions [8] Photoconversion of these two bacteriophytochromes from their Pfr to Pr form triggers the synthesis of the entire photosynthetic apparatus and the associated bacteriochlorophyll and carotenoid molecules [8, 11, 12]. The recent sequencing of the complete genome of Rps. palustris strain CGA009 revealed, in addition to RpBphP1, the unexpected presence of five other putative bacteriophytochrome genes scattered over the genome [16] This suggests that this bacterium has developed a sophisticated and complex network of photoreceptors for its adaptation to light environment. What is the significance of the tandem organization of the two bacteriophytochromes genes? Do these two bacteriophytochromes possess different properties? Are these two bacteriophytochromes involved in the regulation of the closely located pucBAd genes or in different regulatory processes? In this report, we show by combining genetics, biochemical and biophysical approaches that the bacteriophytochrome RpBphP3 possesses unusual photochemical properties and works in tandem with RpBphP2 to regulate the synthesis of the LH4 complexes

Methods

Results

Conclusion