Possible involvement of Ycf33 in cyclic electron transport in Synechocystis sp. PCC6803

Abstract

Ycf (hypothetical chloroplast frame) 33 is a small (7.5 kDa) membrane protein with two membrane spanning helices. It is found in chloroplast genomes of "red" lineages (red algae, diatom and cryptomonad) and a cyanelle genome of a glaucocystophyte (Cyanophora paradoxa). The frame is also found in the nuclear genome of A. thaliana with a chloroplast-targeting signal peptide. It is thus suggested that the gene product is present in all the chloroplasts of photosynthetic organisms. We have disrupted the gene (ssl1417) in Synechocystis sp. PCC6803 and characterized the mutant. The mutant grows normally as does the wild type under moderate light intensity (100 µE/m²·sec). However, the absorption due to phycobilisome compared with chlorophyll is higher than that of wild type. The rereduction rate of P700 in the presence of DCMU was not much different between the mutant and wild type. However, when DPI (diphenyleneiodinium) is present together with DCMU, rereduction of P700 was retarded in the mutant while it was not affected in the wild type. These results indicate that although the rate of cyclic electron transport to P700 is not much different, however, the path of electrons is different between the mutant and wild type. It is suggested that Ycf33 is functioning in cyclic electron transport in wild type, while the path is replaced by another electron carrier(s) which is inhibited by DPI in ycf33 deletion mutant.