Dissection of Nutrient Deprivation Responses in Cyanobacteria and the Degradation of the Light-Harvesting Complex

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9727836 Grossman Phycobilisomes are multiprotein complexes that harvest most of the light energy that is used to drive photosynthesis in cyanobacteria and eukaryotic red algae. When cyanobacteria are deprived of an essential macronutrient, the organism undergoes a suite of responses, some of which are specific to the limiting nutrient (e.g. elevated expression of transport systems that help in the acquisition of the limiting nutrient), while others are general and occur during a number of different stress conditions (e.g. the degradation of phycobilisomes). There is little knowledge concerning the regulatory elements that control the general and specific responses; they may have any intersecting control points. To isolate mutants with defects in the regulation of the general responses, a visual screen to identify strains unable to degrade their phycobilisomes during sulfur deprivation was used. Many of the mutants had a lesion in a gene, designated nblA (for nonbleaching), that encodes a polypeptide of 59 amino acids which is essential for phycobilisome degradation. Six additional nonbleaching mutants which do not have lesions in nblA have been identified. Four of the mutants do not express nblA during nutrient limitation and the remaining two are downstream of the NblA polypeptide in the pathway leading to protein degradation. All of the mutants that do not express nblA are complemented by a gene that encodes a response regulator typical of two-component regulatory systems. Four distinct screens to isolate additional mutants in the regulation of the general responses and the sulfur stress-specific responses are being used. One of the screens would also allow the isolation of additional mutants defective in elements downstream of NblA in the pathway leading to phycobilisome degradation; this screen could identify the proteins with which NblA interacts and the mechanism(s) that lead to the rapid degradation of the multisubunit light-harvesting complex. It is important to elucidate the function of NblA in the bleaching process since, while it is understood how critical proteolysis is for the eliminating denatured, improperly folded and unassembled proteins and for controlling developmental processes and regulatory cascades, little is known about the mechanisms that govern proteolysis and the ways in which specific protein complexes are targeted for degradation.