FUNCTIONALLY IMPORTANT DOMAINS OF THE LARGE HYDROPHILIC LOOP OF CP47 AS PROBED BY OLIGONUCLEOTIDE-DIRECTED MUTAGENESIS IN SYNECHOCYSTIS SP PCC-6803

The chlorophyll a-binding protein CP47 serves as core antenna to photo system II (PS II). The predicted topology of CP47 exhibits six membran e-spanning regions and a large hydrophilic loop (loop E) which rougly includes 200 residues (255-455) and is presumably exposed to the lumen al side of the thylakoid membrane. Several lines of experimental evide nce suggest that loop E might be involved in binding or stabilizing fu nctional manganese in the catalytic site of water oxidation or in inte racting with the extrinsic PS II-O protein (the 33-kDa manganese-stabi lizing protein). To scan loop E for functionally important domains, ol igonucleotide-directed mutagenesis has been used to introduce deletion s of 3-8 residues in conserved and charged regions of loop E. In addit ion, one single-site mutation of the only histidine present in loop E was created (H343L). Domains deleted in DELTA1 (1265-F268), DELTA2 (T2 71-K277), DELTA4 (T304-L309), DELTA5 (F311-N317), and DELTA12 (D440-P4 47) are required for stable assembly of functional PS II complexes. De letion of domains DELTA3 (K277-E283) and DELTA11 (R422-E428) significa ntly reduces the level of assembled PS II and impairs photoautotrophic growth and oxygen evolution. Deletion of domain DELTA8 (A373-D380) en hances the susceptibility to photoinhibition. In contrast, deletion of domains DELTA6 (G333-1336), DELTA7 (K347-R352), DELTA9 (V392-Q394), a nd DELTA10 (D416-F420) and mutation of H343 to leucine do not seem to severly interrupt PS II structure and function, although all mutants e xhibit a slightly decreased stability of PS II as compared to the wild type. Thus, selected domains of the large hydrophilic loop of CP47 ar e important for PS II structure and function. With respect to possible sites of interaction between loop E of CP47 and the extrinsic PS II-O protein, our results indicate that none of the deletions in the regio n from residue 330 to 420 (DELTA6, DELTA7, DELTA8, DELTA9, DELTA10) co mpletely interrupts a functional association of the manganese-stabiliz ing protein to PS II, although the binding characteristics might be ch anged in some cases.