The chlorophyll-binding protein CP47 in photosystem II. Final report

Abstract

Generally, light-harvesting chlorophyll-binding proteins (LHCP) of the Cab family that are prevalent antenna systems in plants are thought to be absent in cyanobacteria. Therefore, it often is tacitly assumed that in cyanobacteria all chlorophyll is associated with the PS II and PS I core antenna. For this reason, it was of interest to investigate what the effect would be of genetic deletion of both the PS I core complex and the PS II core antenna in Synechocystis. Therefore, a mutant was made in which the psaAB genes for the PS I core were deleted, in addition to deletion or inactivation of psbB and/or psbC (coding for CP43). In this series of mutants, also apcE was deleted. In the absence of both CP47 and CP43, also the PS II reaction center proteins D1 and D2 were not detectable in the thylakoid membrane. Thus, both PS II and PS I were deleted in the resulting strains. Nonetheless, a significant amount of chlorophyll (about 15% of that present when PS II was left intact) was found to remain in the PS I-less, psbB{sup {minus}}, psbC{sup {minus}}, apcE{sup {minus}} mutant. This chlorophyll had fluorescence characteristics resembling those of LHC II in higher plants, with a 678 nm emission maximum at 77 K. The properties of this chlorophyll remaining in the absence of PS II and PS I in Synechocystis did not resemble those of chlorophyll bound to a CP43-like protein that has been found in cyanobacteria and that is expressed under iron-stress conditions. However, some similarities in terms of fluorescence emission were observed with the isolated 22 kDa protein encoded by psbS. The role and association of the remaining chlorophyll in the PS I-less, psbB{sup {minus}}, psbC{sup {minus}}, apcE{sup {minus}} mutant remains unclear, however, this chlorophyll protein is expected to be functionally connected to PS II when this photosystem is present.