Excited states and primary photochemical reactions in the photosynthetic bacterium Heliobacterium chlorum

Abstract

The charge separation and excited states of antenna bacteriochlorophyll in membrane fragments of the recently discovered photosynthetic bacterium Helobacterium chlorum were studied by absorbance-difference spectroscopy. Formation ofsinglet excited states of bacteriochlorophyll g with a lifetime of 200 ps or less was observed as the disappearance of the ground state absorption bands. From the absorbancedifference spectra, it was concluded that the primary photochemical reaction consists of the transfer of an electron from the primary donor P-798 to a possibly bacteriochlorophyll c-like pigment absorbing at 670 nm. Electron transfer to the secondary acceptor occurred with a time constant of about 500 ps. The midpoint potential of this acceptor (between -450 and -560 mV) and the absence of sigifcant absorbance changes in the near-infrared upon its reduction suggest that this acceptor is an iron-sulfur center. It is concluded that the primary photochemistry of H. chlorum is similar to that of green sulfur bacteria.The recently discovered photosynthetic bacterium Heliobacterium chlorum (1) has an unusual pigment composition and structure. Instead of bacteriochlorophyll (BChl) a or b, it contains BChl g (2) and a pigment absorbing at 670 nm, and it does not appear to possess either chlorosomes, like green bacteria, or invaginations of the cytoplasmic membrane, like purple bacteria. Nothing is known about its mechanism of photosynthetic electron transport, except that illumination results in (I) the bleaching of a pigment absorbing at 798 nm that may serve as primary electron donor and (ih) the oxidation of a c-type cytochrome (3).Recently we have shown that picosecond absorptiondifference spectroscopy of isolated membranes of the green sulfur bacterium Prosthecochloris aestuarii provides information not only on the properties of the excited states generated in the antenna but also on the components involved in the primary charge separation in this bacterium (4). In the present communication we report the results of a picosecond study of membranes of H. chlorum. Evidence will be presented that the primary photochemistry in this bacterium is similar to that in green sulfur bacteria.MATERIALS AND METHODS H. chlorum was grown anaerobically in medium 112 of the American Type Culture Collection (1) containing 2.5 mM ascorbate. Cells were harvested by centrifugation and resuspended in a buffer of pH 8.0 containing 10 mM Tris, 10 mM ascorbate, and 2 mM dithiothreitol. Membrane fragments were prepared by sonication followed by centrifugation for 15 min at 27,000 x g to remove large cell fragments. The preparation was stored in the dark at 50C before use.For measurements of absorption changes under continuous illumination, the samples were used without further additions. The absorbance changes were measured as in ref.5. Suitable interference and absorbance filters were used to select the actinic illumination and to protect the photomultiplier from stray actinic light. The picosecond absorbancedifference measurements wer...

Protein sequences and redox titrations indicate that the electron acceptors in reaction centers from heliobacteria are similar to Photosystem I

“…The experimental results presented here are in good agreement with the previous conclusion that the photosynthetic RCs from heliobacteria are analogous to PSI of oxygen evolving photosynthetic organisms (Prince et al 1985, Nuijs et al 1985, Brok et al 1986, Trost and Blankenship 1989. The strongest support for this conclusion 17 comes from our sequence data (Fig.…”

“…The antenna pigments are not easily removed without damaging the reaction center. Heliobacteria have only a small antenna of 30-60 antenna pigments per primary donor, P800, which are associated directly with the reaction center core (Nuijs et al 1985). Apparently, they do not contain any other antenna pigment-proteins (Trost andBlankenship 1989, Amesz 1991) and excitations do not appear to be shared between RCs (Deinum et al 1991).…”

Primary photosynthetic processes in Heliobacterium chlorum at 15K

“…The optical difference spectrum of P-798 photooxidation shows a negative band in the Qy region with a maximum near 798 nm at room temperature (Fuller et al 1985, Prince et al 1985, Nuijs et al 1985b). The maximum bleaching shifts to 793 nm at 5 K (Smit et al 1989).…”

Bacteriopheophytin g : Properties and some speculations on a possible primary role for bacteriochlorophylls b and g in the biosynthesis of chlorophylls

“…We have also studied triplet Bpheog (3Bpheog) and the cation free radical BPheog' by ESR. Because of the high current interest in the structure of bacterial photosynthetic reaction centers (6,7), the reaction center of H. chlorum has quickly attracted attention (8)(9)(10). The extreme sensitivity of BChlg to light and air was not adequately taken into consideration in the early reaction center work, and it will be necessary to do so in future reaction center research on this organism.…”