REGULAR PAPER The PucC protein of Rhodobacter capsulatus mitigates an inhibitory effect of light-harvesting 2 a and b proteins on light- harvesting complex 1 Paul R. Jaschke Æ Heidi N. LeBlanc Æ Andrew S. Lang Æ J. Thomas Beatty Received: 7 September 2007 / Accepted: 10 September 2007 / Published online: 9 October 2007 Ó Springer Science+Business Media B.V. 2007 Abstract Rhodobacter capsulatus contains lhaA and pucC genes that have been implicated in light-harvesting complex 1 and 2 (LH1 and LH2) assembly. The proteins encoded by these genes, and homologues in other photosynthetic organisms, have been classified as the bacteriochlorophyll delivery (BCD) family of the major facilitator superfamily. A new BCD family phylogenetic tree reveals that several PucC, LhaA and Orf428-related sequences each form sep- arate clusters, while plant and cyanobacterial homologues cluster more distantly. The PucC protein is encoded in the pucBACDE superoperon which also codes for LH2 a (PucA) and b (PucB) proteins. PucC was previously shown to be necessary for formation of LH2. This article gives evidence indicating that PucC has a shepherding activity that keeps the homologous a and b proteins of LH1 and LH2 apart, allowing LH1 to assemble properly. This shepherding function was indicated by a 62% reduction in LH1 levels in DLHII strains carrying plasmids encoding pucBA along with a C-terminally truncated pucC gene. More severe reductions in LH1 were seen when the truncated pucC gene was co- expressed in the presence of C-terminal PucC::PhoA fusion proteins. It appears that interaction between truncated PucC::PhoA fusion proteins and the truncated PucC protein disrupts LH1 assembly, pointing towards a PucC dimeric or multimeric functional unit. Keywords Photosynthesis Á Purple bacteria Á Light-harvesting Á LH1 Á LH2 Á Rhodobacter Á PucC Á LhaA Á Bacteriochlorophyll delivery homologues Á Complex-specific assembly factors Abbreviations BCD Bacteriochlorophyll delivery BChl Bacteriochlorophyll a ICM Intracytoplasmic membrane system LH1 Light-harvesting complex 1 LH2 Light-harvesting complex 2 RC Reaction centre Introduction Purple photosynthetic bacteria such as Rhodobacter cap- sulatus contain light-harvesting complex 1 (LH1), which surrounds the reaction centre (RC) as part of the core complex, and light-harvesting complex 2 (LH2), which is present in multiple copies of a ring-shaped structure that interconnect core complexes (Bahatyrova et al. 2004; Pa- piz et al. 1996). These complexes are located within differentiated invaginations of the cytoplasmic membrane called the intracytoplasmic membrane system (ICM), which is formed in response to oxygen deprivation (Drews and Golecki 1995). The bacteriochlorophyll a (BChl) pigments of each LH complex have different Q y (far-red) absorption maxima: BChl in the LH1 complex has a far-red P. R. Jaschke Á H. N. LeBlanc Á J. T. Beatty (&) Department of Microbiology and Immunology, University of British Columbia, 2350 Health Sciences Mall, Vancouver, BC, Canada V6T 1Z3 e-mail: jbeatty@interchange.ubc.ca Present Address: H. N. LeBlanc Medarex, Inc, 521 Cottonwood Dr, Milpitas, CA 95035, USA A. S. Lang Department of Biology, Memorial University of Newfoundland, St. John’s, NL, Canada A1B 3X9 123 Photosynth Res (2008) 95:279–284 DOI 10.1007/s11120-007-9258-x