Phycobilisome degradation and responses of cyanobacteria to nutrient limitation and high light

Microorganisms must sense their environment and rapidly tune their metabolism to ambient conditions to efficiently utilize available resources. We have developed screens to isolate mutants unable to degrade their light-harvesting complex (phycobilisome) in response to nutrient deprivation. Complementation of these nonbleaching mutants has allowed us to uncover several genes involved in the pathway of sensing and responding to nutrient deprivation. One of these non-bleaching mutants has a lesion in a gene that is designated nblA encoding a small polypeptide that has homology to putative polypeptides in other cyanobacteria and red algae. No catalytic function for this polypeptide has been established although it is absolutely required for phycobilisome degradation A second mutant is complemented by the nblB gene, which encodes a polypeptide with homology to the family of proteins involved in attaching chromophores to apophycobiliprotein subunits. This protein may be required for removing bilin chromophores from phycobiliprotein subunits prior to their degradation. A third mutant was complemented by the nblR gene. NblR is a response regulator that appears to control at least some of the 'general' responses that occur during any of a number of different stress conditions. It is required Mr degradation of phycobilisomes and also appears to be necessary for controlling changes in photosynthetic activity that occur during exposure of cells to either nutrient limiting conditions or high light; this control is critical for allowing the cells to survive adverse environmental conditions.