Exploring the ability of chlorophyll b to bind to the CP43' protein induced under iron deprivation in a mutant of Synechocystis PCC 6803 containing the cao gene.
Cyanobacteria, unlike plants and green algae, do not contain chlorophyll (Chl) b. This is because of the absence of the cao gene which encodes the enzyme that catalyses a two step oxygenation of chlorophyllide a to chlorophyllide b. Recently, however, the cao gene of higher plants was engineered into Synechocystis PCC 6803 leading to Chl b synthesis in this cyanobacterium [Satoh et al., J. Biol. Chem. 276 (2001) 4293-4297]. Here we use this same cao-plus mutant to show that Chl b can bind to the CP43' protein, expressed in cells exposed to low iron levels, which normally binds Chl a only. In so doing CP43' is changed to a Chl a/Chl b-binding protein and in this respect resembles the closely related Chl a/Chl b-binding Pcb protein of prochlorophytes (green oxyphotobacteria). The results emphasise the possibility of using an in vitro system to elucidate factors which control the binding of these two different forms of chlorophylls to the six transmembrane helical light-harvesting proteins of oxygenic photosynthetic organisms.