SUPPRESSION OF MUTANTS ABERRANT IN LIGHT-INTENSITY RESPONSES OF COMPLEMENTARY CHROMATIC ADAPTATION

Complementary chromatic adaptation is a process in which cyanobacteria alter the pigment protein (phycocyanin and phycoerythrin) composition of their light-harvesting complexes, the phycobilisomes, to help opti mize the absorbance of prevalent wavelengths of light in the environme nt. Several classes of mutants that display aberrant complementary chr omatic adaptation have been isolated. One of the mutant classes, desig nated ''blue'' or FdB, accumulates high levels of the blue chromoprote in phycocyanin in low-intensity green light, a condition that normally suppresses phycocyanin synthesis. We demonstrate here that the synthe sis of the phycocyanin protein and mRNA in the FdB mutants can be supp ressed by increasing the intensity of green light. Hence, these mutant s have a decreased sensitivity to green light with respect to suppress ion of phycocyanin synthesis. Although we were unable to complement th e blue mutants, we did isolate genes that could suppress the mutant ph enotype. These genes, which have been identified previously, encode a histidine kinase sensor and response regulator protein that play key r oles in controlling complementary chromatic adaptation. These findings are discussed with respect to the mechanism by which light quality an d quantity control the biosynthesis of the phycobilisome.