The regulatory mechanisms for maintaining the chlorophyll to apoproteins ratio.

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When etiolated seedlings are illuminated,chlorophyll(Chl)begins to be synthesized with accumulation of Chl-protein complex(CP)apoproteins. Newly synthesized Chl is incorporated into apoproteins to form CPs,which are associated with other components to form functional photosystems.Each CP is supposed to be composed of Chl and its apoprotein in a definite ratio. It have been shown that light-harvesting Chl a/b-protein complex of PSII binds 15 Chl. The Chl/P700 ratio for P700-Chl a-protein complex isolated by SDS-PAGE was reported to be 120. Two apoproteins of PSII core have definite ratios of Chl to apoproteins Taking into consideration the observation that all the Chl molecules bind to apoproteins in thylakoid membranes,these indicate that the ratio of Chl to apoproteins of mature chloroplasts would be kept constant.Accumulation of excess Chl or apoproteins which do not bind with each other would not be favorable for chloroplasts. The accumulation of apoproteins must be regulated in accordance with changes in the rate of Chl synthesis. We focussed our attention on the regulatory mechanisms operate to maintain the Chl to apoprotein ratio constant during chloroplast development and the followings were elucidated.(1)Chl is supplied to apoproteins in two ways:by distribution of newly synthesized Chl and by redistribution of Chl that have been previously incorporated into other CPs. (2)The formation of different CPs is regulated by differential affinities of the apoproteins for Chl.(3)Coordinate accumulation of Chl and apoproteins of CPs is achieved through rapid digestion of apoproteins which are not bound with Chl.