Electron transport in Heliobacterium chlorum wholecells studied by electroluminescence and absorbance difference spectroscopy

An electric field pulse induced luminescence signal was observed in suspensions of whole cells of the photosynthetic green bacterium Heliobacterium chlorum. This signal is associated with electron transport in the reaction center. The dependence of the electroluminescence on the delay between the flashand the pulse and the absorbance kinetics at 800 and 553 nm revealed the following properties of electron transport in whole cells: (1) most of P-798⁺ was rereduced by cytochrome c-553 in two phases with half-times of 0.11 and 0.7 ms and equalamplitudes (this is considerably faster than the timesreported for membrane fragments); (2) if reduced cytochrome c-553 is not available, P-798⁺ reacts back with a reduced acceptor in 20 ms; (3) oxidized cytochrome c-553 is rereduced in about 20 ms, presumably by an external donor, because it does not take place after resuspension of the cells in Tris buffer. In membrane fragments, spheroplasts and cells stored at -20°C, cytochrome oxidation is inhibited and no electroluminescence can be observed.