A pufX gene deletion in the purple bacterium Rhodobacter capsulatus causes a severe photosynthetic defect and increases core light-harvesting complex (LH1) protein and bacteriochlorophyll a (BChl) levels. It was suggested that PufX interrupts the LH1 alpha/beta ring around the reaction centre, allowing quinone/quinol exchange. However, naturally PufX(-) purple bacteria grow photosynthetically with an uninterrupted LH1 We discovered that substitutions of the Rhodobacter-specific LH1 alpha seryl-2 decrease carotenoid levels in PufX(-) R. capsulatus. An LH1 alpha S2F mutation improved the photosynthetic growth of a PufX- strain lacking the peripheral LH2 antenna, although LH1 BChl absorption remained above wild-type, suggesting that Rhodobacter-specific carotenoid binding is involved in the PufX(-) photosynthetic defect and LH1 expansion is not. Furthermore, PufX overexpression increased LH1-like BChl absorption without inhibiting photosynthetic growth. PufX(+) LH1 alpha S2-substituted mutant strains had wild-type carotenoid levels, indicating that PufX modulates LH1 carotenoid binding, inducing a conformational change that favours quinone/quinol exchange. (c) 2005 Elsevier Inc. All rights reserved.