Archives of Biochemistry and Biophysics 437 (2005) 186–198 www.elsevier.com/locate/yabbi 0003-9861/$ - see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.abb.2005.03.012 The puhE gene of Rhodobacter capsulatus is needed for optimal transition from aerobic to photosynthetic growth and encodes a putative negative modulator of bacteriochlorophyll production Muktak Aklujkar a , Roger C. Prince b , J. Thomas Beatty a,¤ a Department of Microbiology and Immunology, University of British Columbia, 300-6174 University Blvd., Vancouver, BC, Canada V6T 1Z3 b ExxonMobil Research and Engineering Co., Annandale, NJ 08801, USA Received 2 February 2005, and in revised form 10 March 2005 Available online 25 March 2005 Abstract A conserved orf of previously unknown function (herein designated as puhE) is located 3' of the reaction centre H (puhA) gene in purple photosynthetic bacteria, in the order puhABCE in Rhodobacter capsulatus. Disruptions of R. capsulatus puhE resulted in a long lag in the growth of photosynthetic cultures inoculated with cells grown under high aeration, and increased the level of the peripheral antenna, light-harvesting complex 2 (LH2). The amount of the photosynthetic reaction centre (RC) and its core antenna, light-harvesting complex 1 (LH1), was reduced; however, there was no decrease in expression of a lacZ reporter fused to the puf (RC and LH1) promoter, in RC assembly in the absence of LH1, or in LH1 assembly in the absence of the RC. In strains that lack LH2, disruption of puhE increased the in vivo absorption at 780 nm, which we attribute to excess bacteriochlorophyll a (BChl) pigment production. This eVect was seen in the presence and absence of PufQ, a protein that stimulates BChl biosynthesis. Expression of puhE from a plasmid reduced A 780 production in puhE mutants. We suggest that PuhE modulates BChl biosynthesis independently of PufQ, and that the presence of excess BChl in PuhE ¡ LH2 + strains results in excess LH2 assembly and also interferes with the adap- tation of cells during the transition from aerobic respiratory to anaerobic photosynthetic growth.  2005 Elsevier Inc. All rights reserved. Keywords: Photosynthesis; Purple bacteria; Light-harvesting; Reaction centre; Rhodobacter; Bacteriochlorophyll Rhodobacter capsulatus is a purple nonsulfur bacte- rium that grows photosynthetically under anaerobic illu- minated conditions and represses the expression of photosynthesis genes when growing by aerobic respira- tion [1]. In response to oxygen deprivation, purple photosynthetic bacteria assemble a photosynthetic apparatus within diVerentiated invaginations of the cytoplasmic membrane called the intracytoplasmic membrane system (ICM) [2]. This apparatus includes three membrane-bound pigment–protein complexes: the reaction centre (RC), which functions as a light-depen- dent quinone reductase; light-harvesting (LH) complex 1, which is adjacent to and forms a ring or arc encircling the RC as part of the core complex that includes the PufX polypeptide [3–9]; and the LH2 complex, which is present in multiple copies of a ring-shaped structure that are peripheral to the core complex [10,11]. The presence of these photosynthetic complexes can be evaluated by their characteristic light absorption spectra, which reXect the protein environment around bacteriochlorophyll a (BChl): protein-free BChl has a far-red absorption peak at »780 nm, whereas the LH2 BChl peaks of R. capsula- tus are at »800 and 850 nm, and BChls in the LH1 com- plex absorb light of approximately 870 nm. The far-red absorption peaks of the RC are at about 760 nm (bacte- riopheophytins), 804 nm (accessory or ‘voyeur’ BChls), and 865 nm (the ‘special pair’ of BChls) [12]. * Corresponding author. Fax: +1 604 822 6041. E-mail address: jbeatty@interchange.ubc.ca (J.T. Beatty).