ENERGY-DISTRIBUTION BETWEEN PHOTOSYSTEM-I AND PHOTOSYSTEM-II IN THE PHOTOSYNTHETIC PROKARYOTE PROCHLOROTHRIX-HOLLANDICA INVOLVES A CHLOROPHYLL-A B ANTENNA WHICH ASSOCIATES WITH PHOTOSYSTEM-I/

Prochlorothrix hollandica is a photosynthetic prokaryote, whose main t hylakoid intrinsic chlorophyll a/b antenna copurifies with PS I and is both structurally and functionally distinct from chloroplast LHC II. The 35 kDa apoprotein of the antenna forms the main target for light/r edox controlled reversible phosphorylation (Post, A.F., Gal, A., Ohad, I., Milbauer, K.M. and Bullerjahn, G.S. (1992) Biochim. Biophys. Acta 1100, 75-82). The occurrence of state 1 --> 2 transitions in cells il luminated with light 1 (710 nm) and light 2 (652 nm) was shown from di fferences in fluorescence properties using the chlorophyll fluorescenc e induction technique. The same technique showed that the redox state of the PQ pool responded to light conditions, being more oxidized in d ark-incubated and light-1-illuminated cells. Following the transfer of light-1-treated cells to light 2 conditions, state 2 was reached in a pprox. 10 min. Addition of the phosphatase inhibitor NaF locked cells in state 2. These observations lend support to the hypothesis that the molecular mechanism driving the state 1 --> 2 transitions involves th e reversible phosphorylation of the main chlorophyll a/b antenna. 77 K fluorescence spectra of whole cells and of PS I complexes obtained fr om detergent-treated thylakoids showed strong energy coupling between the antenna and PS I preferentially. Fluorescence quenching measuremen ts showed an increase in PS I activity during a state 1 --> 2 transiti on. These observations suggest that during a state 1 --> 2 transition an increasing fraction of the excitation energy arriving from the ante nna is diverted to PS I. The antenna copurified with PS I complexes un der all conditions examined. We have summarized the principal differen ces between eukaryotic and prokaryotic chlorophyll a/b antennae. A mod el for the regulation of photosynthetic activity in P. hollandica is p roposed and it involves light controlled reversible phosphorylation of the chlorophyll a/b antenna. Our model claims that in state 1 (non-ph osphorylating conditions) the bulk chlorophyll a/b antenna is shared b y both photosystems. Under phosphorylating conditions (state 2) the an tenna associates more tightly with PS I, effectively reducing energy t ransfer to PS II centers. It is noted that reversible phosphorylation of a chlorophyll a/b antenna may have evolved as a regulatory mechanis m prior to the evolution of chloroplast LHC II.