A response regulator of cyanobacteria integrates diverse environmental signals and is critical for survival under extreme conditions

Abstract

Microorganisms must sense their environment and rapidly tune their metabolism to ambient conditions to efficiently use available resources. We have identified a gene encoding a response regulator, NblR, that complements a cyanobacterial mutant unable to degrade its light-harvesting complex (phycobilisome), in response to nutrient deprivation. Cells of the nblR mutant (i) have more phycobilisomes than wild-type cells during nutrient-replete growth, (ii) do not degrade phycobilisomes during sulfur, nitrogen, or phosphorus limitation, (iii) cannot properly modulate the phycobilisome level during exposure to high light, and (iv) die rapidly when starved for either sulfur or nitrogen, or when exposed to high light. Apart from regulation of phycobilisome degradation, NblR modulates additional functions critical for cell survival during nutrient-limited and high-light conditions. NblR does not appear to be involved in acclimation responses that occur only during a specific nutrient limitation. In contrast, it controls at least some of the general acclimation responses; those that occur during any of a number of different stress conditions. NblR plays a pivotal role in integrating different environmental signals that link the metabolism of the cell to light harvesting capabilities and the activities of the photosynthetic apparatus; this modulation is critical for cell survival.

Immunolocalization of NblA, a protein involved in phycobilisome turnover, during heterocyst differentiation in cyanobacteria

“…At present NblA proteins have been studied almost exclusively in unicellular non-N 2 -fixing cyanobacteria, for which phycobilisome degradation appears to be a very important adaptative mechanism that improves survival of the cyanobacteria under high light conditions and during nutrient limitation (Görl et al, 1998;Schwarz & Grossman, 1998;Sauer et al, 2001;van Waasbergen et al, 2002). For the heterocyst-forming strains, nitrogen starvation leads to the switching on of the N 2 -fixation process and the shut-off of O 2 evolution which is necessary for the functioning of the nitrogenase.…”

Acclimation of unicellular cyanobacteria to macronutrient deficiency: emergence of a complex network of cellular responses

“…During growth under nutrient-sufficient conditions, the reducing potential produced by the photosynthetic electrontransport chain is used for anabolic reactions. Nutrient limitation slows down the reoxidation of the final electron acceptors, and therefore electron transfer activity must be down-regulated (Grossman et al, 1993;Schwarz & Grossman, 1998). The adjustment of the photosynthetic Fig.…”

NblR Is a Novel One-Component Response Regulator in the CyanobacteriumSynechococcus elongatusPCC 7942

“…4) Phosphorylation of the Asp residue activates its C-terminal effector domain and induces the signal-specific adaptation response. 5) In the cyanobacterium Synechococcus elongatus PCC 7942 (hereafter referred to as S. elongatus), several nbl (non-bleaching) mutants, viz., nblA, 6) nblS, 7) nblR, 8) nblB, 9) and nblC, 10) have been identified. NblR has been identified as a response regulator belonging to the OmpR/PhoR family in two-component systems; under nutrient deprivation conditions, it plays a critical role in the bleaching process via transcriptional activation of nblA.…”

“…NblR has been identified as a response regulator belonging to the OmpR/PhoR family in two-component systems; under nutrient deprivation conditions, it plays a critical role in the bleaching process via transcriptional activation of nblA. 8,11) Since the nblR gene is constitutively expressed when cells are grown in either nitrogen-or sulfur-free (-N, -S) medium, NblR activity might be modified at the post-translational level. 11) A histidine kinase NblS (non-bleaching sensor) might be a potential partner of NblR, because under nutrient-deprivation and high light stress, both nblS and nblR mutants exhibit a similar non-bleaching phenotype, 2,7) but phosphotransfer from NblS to NblR has not been reported to date.…”