PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Bacteriophytochromes control the expression of spectrally distinct light harvesting complexes in Rhodopseudomonas palustris. Miroslav Papiz^*,1, Anthony Fordham-Skelton¹, Katie Evans^{1, 2}, Colin Reynolds², Anna Lawless¹, Hiten Mistry¹, ¹ CCLRC Daresbury Laboratory, Warrington, Cheshire, UK²

ABSTRACT- The genome of the metabolically versatile phototropic bacterium Rhodopseudomonas palustris has revealed the organisation of genes involved in photosynthesis. Unusually the bacterium has 5 peripheral light-harvesting (LH) genes. The most common LH2 complex in purple bacteria has a nonameric structure with near infrared absorption peaks at 800 and 850 nm. Rps palustris is unique in that it adapts to low-light conditions with a large increase in absorption at 800 nm. The genome has also revealed the presence of 6 bacteriophytochromes (Bph). It has been shown, by analogy with Bradyrhizobium sp, that the expression of the main photosynthetic gene cluster (PGC) is controlled by one of these. However this interpretation has been put in doubt by genomic sequencing indicating the PGC Bph to be frame shifted. Our aim is to understand the nature of the LH complex responsible for low-light adaptation and to determine how the expression is controlled. We have shown that the low light adaptation arises from a new complex named LH4 which has a single absorption peak at 800 nm. The complex contains peptides encoded by the pucD genes and comprises 90% of all LH complexes when grown under low-light conditions. The complex is an octamer with 4 bacteriochlorophyll molecules per repeating unit and a very different pigment orientation and excitonic structure compared to LH2. We have expressed and characterised two Bphs important in the control of LH2 and LH4 production. Knowledge of the spectral properties of these Bphs has made it possible to design experiments which demonstrate that specific monochromatic light alone can cause the over-expression of LH4 complexes. The spectral properties of the various LH and Bph molecules and the environment in which Rps. palustris lives suggests an explanation for the low-light adaptation.

KEY WORDS: antenna, gene expression, Bacteriophytochrome, acclimation