The orientation of the long-wavelength (Q y) transition moments of the antenna bacterioviridin (BVr) was examined in living cells of Chlorobium limicola. Previous linear dichroism studies [(1986) FEBS Lett. 199, 234–236] indicated that in each individual chromatophore of C. limicola the Q y, transition moment vectors of the whole chlorosome BVr are essentially parallel to each other and are practically ideally oriented along the long axis of the chlorosome. We measured the picosecond polarized fluorescence decay kinetics for antenna bacteriochlorophyll (BChl) emissions upon selective excitation with polarized light of the Q y, transition of BVr. The polarization (p) of the BVr fluorescence is measured to be constant during the BVr excited-state lifetime and to be equal to the limiting value of p achieved in monomeric BChl: P = + 0.42 ± 0.02. The results indicate convincingly that the excitation energy transfer within chlorosomes of C. limicola cells takes place between chromophores (or their coupled associates) with parallel transition moment vectors.
