Studies on photosystem II in higher plant chloroplasts

Abstract

The organisation of P511 components of a higher plant (lettuce) has been investigated using chloroplasts and isolated 0 2—evolving PSII preparations. Both energy transfer and molecular aspects of P511 have been investigated. The initial reactions of energy transfer that occur within the first few nanoseconds of excitation were measured by time resolved fluorescence using the technique of single photon counting. Fluorescence lifetimes from lettuce chloroplasts and P511
particles prepared from these chloroplasts were measured along with PSI preparations from Chlorogloea fritschii. PSI from lettuce were too unstable for our purposes. It was hoped to obtain an insight into the light—harvesting chlorophyll—reaction centre organisation and energy transfer mechanisms by this technique. The chlorophyll protein complexes of the whole chloroplast system is very complex and so in order to simplify the situation isolated PSI and PSII preparations were used. It was shown that in chloroplasts there are at least three fluorescing components and that they originate mainly from the PSII
complex. These components are characterised by their lifetimes into fast, middle and long fluorescing components of approximately 100, 500 and 1500 psec respectively. The fast and long fluorescing components seem to be closely coupled to the reaction centre as the proportions of these two components are dependent on the redox state of the reaction centre. The contribution by the fast component increases whilst that of the long component decreases in the presence of an electron acceptor. The reverse occurs when the reaction centres are closed by DCMIJ. The middle component seems to be a component loosely coupled to the reaction centre and could be mobile PSIIPSII light harvesting chlorophylls. The differing lifetimes of these components could reflect the relative sizes of the chlorophyll pools. Our P811 data was analysed employing a number of currently proposed models, in particular the bipartite model used by Schatz and Holzvarth (1986). However, none of the models applied to our data could adequately explain them. The lifetime data from PSI was dominated by a 20 psec component which was not seen when the chloroplast measurements were taken. Thus a lot more work is needed
to be dane before energy transfer reacti one within the chlorophyll complexes can be fully understood.
On the molecular aspects of P811 organisation a calcium—binding protein (CaRP) has heen isolated from this complex. This CaBP shows similarities to a well characterised regulator CaRP called calmodulin. Thus the isolated protein could be involved in regulation of some aspect of the photosynthetic apparatus.
Evidence has also been gathered implicating Ca2+ as being involved in the OEC possibly in a structural role to organise the components into an efficient 02—evolving configuration. It would also seem that as the isolated P811 preparation ages it undergoes a series of conforniational changes which alters the system. The response to the addition of Pb2 ' in the presence of Ca 2t. This could have significant implications for experimental protocols when using isolated P811 particles.