IMPACT OF STRESS CONDITIONS ON THE PHOTOSYNTHETIC MACHINERY

Abstract

Organisms sense their environment and attempt to tune their metabolism to ambient conditions to efficiently utilize available resources. Cyanobacteria under conditions of stress (nutrient limitation, excess light) modify the photosynthetic apparatus by eliminating most photosystem II activity and degrading their light‐harvesting complex or phycobilisomes (PBS). We have identified cyanobacterial mutants that cannot acclimate normally to nutrient deprivation. One of these mutants has a lesion in a gene encoding a small polypeptide designated NblA that appears to target the PBS for degradation during nutrient limitation. A second lesion is in the nblB gene, which encodes a polypeptide with homology to a family of lyases involved in attaching chromophores to apophycobiliproteins. This protein may remove bilin chromophores from phycobiliprotein subunits prior to their degradation. A third mutant is defective in nblR, which encodes a transcription factor that controls some of the “general” responses that occur during any of a number of different stress conditions. It is required for degradation of PBS and appears to be necessary for controlling photosynthetic activity during both nutrient limitation and high light conditions; this control is critical for survival of cells during conditions of environmental stress. A fourth mutant is altered in nblS, which encodes a sensor kinase that plays a role in regulating both nutrient stress and high light responses. This sensor protein has a PAS domain, appears to binds a flavin, and integrates the response of the cell to a variety of stress conditions; it may sense the redox status of the cell. I will present a model that describes how cyanobacteria sense and respond to environmental change.