Rod substructure in cyanobacterial phycobilisomes: analysis of Synechocystis 6701 mutants low in phycoerythrin.

Abstract

Synechocystis 6701 phycobilisomes contain phycoerythrin, phycocyanin and allophycocyanin in a molar ratio of .apprx. 2:2:1, and other polypeptides of 99-, 46-, 33.5-, 31.5-, 30.5- and 27-kdaltons. Wild-type phycobilisomes consist of a core of 3 cylindrical elements in an equilateral array surrounded by a fanlike array of 6 rods each made up of 3-4 stacked disks. Twelve nitrosoguanidine-induced mutants were isolated which produced phycobilisomes containing between 0-53% of the wild-type level of phycoerythrin and grossly altered levels of the 30.5- and 31.5-kdalton polypeptides. Assembly defects in these mutant particles were shown to be limited to the phycoerythrin portions of the rod substructures of the phycobilisome. Quantitative analysis of phycobilisomes from wild-type and mutant cells, grown either in white light or chromatically adapted to red light, indicated a molar ratio of the 30.5- and 31.5-kdalton polypeptides to phycoerythrin of 1:6, i.e., one 30.5- or one 31.5-kdaltons polypeptide/(.alpha..beta.)6 phycoerythrin hexamer. Presence of the phycoerythrin-31.5-dkalton complex in phycobilisomes did not require the presence of the 30.5-kdalton polypeptide. The converse situation was not observed. These and earlier studies show that the average rod in wild type Synechocystis 6701 phycobilisomes consists of 4 stacked disk-shaped complexes: phycocyanin (.alpha..beta.)6-27 kdalton, phycocyanin (.alpha..beta.)6-33.5 kdalton, phycoerythrin (.alpha..beta.)6-31.5 kdalton and phycoerythrin-30.5 kdalton, listed in order starting with the disk proximal to the core.