FURTHER CHARACTERIZATION OF THE CHLOROPLAST GRANA MARGINS - THE NON-DETERGENT PREPARATION OF GRANAL PHOTOSYSTEM-I CANNOT REDUCE FERREDOXIN IN THE ABSENCE OF NADP(+)REDUCTION

The chloroplast grana margins of spinach thylakoids were isolated by s onication and aqueous-two-phase partitioning and their electron transp ort properties examined. Photosystem II and I electron transport activ ities were measured and compared to the appressed and non-appressed gr ana core and stroma lamellae, respectively, as well as to whole thylak oids. The results show that the PS II complexes in the grana margins a re of the PS IIbeta subtype with respect to antenna size, but are Q(B) reducing with respect to the acceptor side properties, while the PS I centers in the grana margins have slightly larger antennae as compare d to the PS I centers in the stroma lamellae and are more like the PS I, centers located in the grana domain. The ability to reduce ferredox in and NADP(+) was also tested and it was found that the grana margin membrane fraction was unable to reduce ferredoxin, even in the presenc e of added artificial electron donors. The stroma lamellae and whole t hylakoid fractions both reduced ferredoxin at high rates. However, the grana margins could catalyze the reduction of NADP(+) when supplied w ith the necessary components (ferredoxin, ferredoxin:NADP(+) oxidoredu ctase, and an electron source). The results suggest that the PS I popu lations located in the margins of the grana domain are functionally di fferent from the PS I centers located in the stroma lamellae. The data support a model whereby the PS I centers in the grana are primarily i nvolved in non-cyclic electron transport, while the PS I centers locat ed in the stroma lamellae are capable of participating in both cyclic and non-cyclic electron transport.