Assembly of photosynthetic apparatus in Rhodobacter sphaeroides as revealed by functional assessments at different growth phases and in synchronized and greening cells.

Abstract

The development of photosynthetic membranes of intact cells of Rhodobacter sphaeroides was tracked by light-induced absorption spectroscopy and induction and relaxation of the bacteriochlorophyll fluorescence. Changes in membrane structure were induced by three methods: synchronization of cell growth, adjustment of different growth phases and transfer from aerobic to anaerobic conditions (greening) of the bacteria. While the production of the bacteriochlorophyll and carotenoid pigments and the activation of light harvesting and reaction center complexes showed cell-cycle independent and continuous increase with characteristic lag phases, the accumulation of phospholipids and membrane potential (electrochromism) exhibited stepwise increase controlled by cell division. Cells in the stationary phase of growth demonstrated closer packing and tighter energetic coupling of the photosynthetic units (PSU) than in their early logarithmic stage. The greening resulted in rapid (within 0-4h) induction of BChl synthesis accompanied with a dominating role for the peripheral light harvesting system (up to LH2/LH1 ~2.5), significantly increased rate (~7·10(4)s(-1)) and yield (F v/F max ~0.7) of photochemistry and modest (~2.5-fold) decrease of the rate of electron transfer (~1.5·10(4)s(-1)). The results are discussed in frame of a model of sequential assembly of the PSU with emphasis on crowding the LH2 complexes resulting in an increase of the connectivity and yield of light capture on the one hand and increase of hindrance to diffusion of mobile redox agents on the other hand.