Formation of Chlorophyll-Protein Complexes during Greening 1. Distribution of Newly Synthesized Chlorophyll among Apoproteins

The relationship between the accumulation of Chl and the apoproteins of the light-harvesting Chl a/b-protein complex of PS II (LHCII) during the greening of cucumber cotyledons was studied. LHCII apoproteins were not detected in etiolated cotyledons. Upon illumination, Chl a was formed as a result of photoconversion of protochlorophyllide (Pchlide) which had accumulated in the dark. During the lag period that preceded the accumulation of Chl, a small amount of LHCII apoproteins appeared. The amount of LHCII apoproteins increased with increases in levels of Chl b, though somewhat more rapidly during the first 10 h of greening. Treatment with benzyladenine (BA) or levulinic acid (LA) was used to vary the supply of Chl a for apoproteins by promoting or inhibiting the synthesis of Chl a, respectively. LA decreased but BA increased the rate of accumulation of Chl b and LHCII apoproteins. Only small amounts of Chl b and LHCII apoproteins were formed under intermittent illumination. However, in the presence of chloramphenicol (CAP), which inhibits the synthesis of plastome-coded proteins including apoproteins of the P700-Chl a-protein complex (CP1) and a Chl a-protein complex of PS II (CPa), we observed the accumulation of Chl b and LHCII apoproteins, both of which are of nuclear origin. During incubation in the dark after intermittent exposure to light, CAP alone allowed neither destruction nor accumulation of Chl b and LHCII apoproteins, but it did enhance the effect of CaCl2 in inducing both Chl b and these apoproteins. These results can be explained by assuming that apoproteins of CP1 and CPa have a higher affinity for Chl a than do LHCII apoproteins. When the availability of Chl a is limited, these apoproteins compete with one another for Chl a, with the resultant preferential formation of CP1 and CPa. However, when the supply of Chl a becomes large enough for saturation of apoproteins of CP1 and CPa, some of the Chl a is incorporated into LHCII apoproteins either directly or after conversion to Chl b. Thus, the formation of different Chl-protein complexes (CPs) is regulated by the relative rates of synthesis of Chl a and apoproteins and by differential affinities of the apoproteins for Chl a.