BACTERIOCHLOROPHYLL ORGANIZATION AND ENERGY-TRANSFER KINETICS IN CHLOROSOMES FROM CHLOROFLEXUS-AURANTIACUS DEPEND ON THE LIGHT REGIME DURING GROWTH

We have used measurements of fluorescence and circular dichroism (CD) to compare chlorosome-membrane preparations derived from the green fil amentous bacterium Chloroflexus aurantiacus grown in continuous cultur e at two different light-intensities. The cells grown under low light (6 mu mol m(-2) s(-1)) had a higher ratio of bacteriochlorophyll (BChl ) c to BChl a than cells grown at a tenfold higher light intensity; th e high-light-grown cells had much more carotenoid per bacteriochloroph yll. The anisotropy of the Qy band of BChl c was calculated from stead y-state fluorescence excitation and emission spectra with polarized li ght. The results showed that the BChl c in the chlorosomes derived fro m cells grown under high light has a higher structural order than BChl c in chlorosomes from low-light-grown cells. In the central part of t he BChl c fluorescence emission band, the average angles between the t ransition dipole moments for BChl c molecules and the symmetry axis of the chlorosome rod element were estimated as 25 degrees and 17 degree s in chlorosomes obtained from the low- and high-light-grown cells, re spectively. This difference in BChl organization was confirmed by the decay associated spectra of the two samples obtained using picosecond single-photon-counting experiments and global analysis of the fluoresc ence decays. The shortest decay component obtained, which probably rep resents energy-transfer from the chlorosome bacteriochlorophylls to th e BChl a in the baseplate, was 15 ps in the chlorosomes from high-ligh t-grown cell but only 7 ps in the preparation from low-light grown cel ls. The CD spectra of the two preparations were very different: chloro somes from low-light-grown cells had a type II spectrum, while those f rom high-light-grown cells was of type I (Griebenow et al. (1991) Bioc him Biophys Acta 1058: 194-202). The different shapes of the CD spectr a confirm the existence of a qualitatively different organization of t he BChl c in the two types of chlorosome.