Chlorophyll b can serve as the major pigment in functional photosystem II complexes of cyanobacteria

An Arabidopsis thaliana chlorophyll(ide) a oxygenase gene (cao), which is r esponsible for chlorophyll b synthesis from chlorophyll a, was introduced a nd expressed in a photosystem I-less strain of the cyanobacterium Synechocy stis sp. PCC 6803. In this strain, most chlorophyll is associated with the photosystem II complex. In line with observations by Satoh et al, [Satoh, S ., Ikeuchi, M., Mimuro, M. & Tanaka, A. (2001) J. Biol. Chem. 276, 4293-429 7], chlorophyll b was made but accounted for less than 10% of total chlorop hyll. However, when Ihcb encoding light-harvesting complex (LHC)II from pea was present in the same strain (Ihcb(+)/cao(+)), chlorophyll b accumulated in the cell to levels exceeding those of chlorophyll a, although LHCII did not accumulate. in the Ihcb(+)/cao(+) strain, the total amount of chloroph yll, the number of chlorophylls per photosystem II center, and the oxygen-e volving activity on a per-chlorophyll basis were similar to those in the ph otosystem I-less strain. Furthermore, the chlorophyll alb ratio of photosys tem II core particles (retaining CP47 and CP43) and of whole cells of the I hcb(+)/cao(+) strain was essentially identical, and PS II activity could be obtained efficiently by chlorophyll b excitation. These data indicate that chlorophyll b functionally substitutes for chlorophyll a in photosystem II . Therefore, the availability of chlorophylls, rather than their binding sp ecificity, may determine which chlorophyll is incorporated at many position s of photosystem II. We propose that the transient presence of a LHCII/chlo rophyll(ide) a oxygenase complex in the Ihcb(+)/cao(+) strain leads to a hi gh abundance of available chlorophyll b that is subsequently incorporated i nto photosystem II complexes. The apparent LHCII requirement for high chlor ophyll(ide) a oxygenase activity may be instrumental to limit the occurrenc e of chlorophyll b in plants to LHC proteins.