The cyanobacterial macromolecular pigment complex,the phycobilisome, is subjected to regulated proteolysis during nutrient starvation. This pivotal acclimation response allows 'recycling of nutrients' and prevents excessive light excitation and its consequent oxidative damage. An essential component of the degradation process is NblB, which exhibits similarity to members of a family ofenzymesinvolved in pigment biogenesis. These enzymescatalyze the covalent attachment of bilinchromophores to the relevant apo-proteins. Inactivation of nblB, however, does not affect pigment synthesis; rather, it impairs pigment degradation. Therefore, it was proposed that the homology of NblB to the pigment synthesizing enzymes reflects its ability to interact with bilinchromophores and that this recognition is crucial for chromophore detachment and further pigment degradation. Using a mutational approach we identified amino acid residues that are essential for NblB function. Structure-function relationship of NblB and the mechanism of disassemblyof the macromolecular pigment complex and chromophore detachment will be discussed.
