Krauspe, V. | bioRxiv Science

bioRxiv2021

Phycobilisome breakdown effector NblD is required to maintain the cellular amino acid composition during nitrogen starvation

Small proteins are critically involved in the acclimation response of photosynthetic cyanobacteria to nitrogen starvation. NblD is the 66-amino-acid effector of nitrogen-limitation-induced phycobilisome breakdown, which is believed to replenish the cellular amino acid pools. To address the physiological functions of NblD, the concentrations of amino acids, intermediates of the arginine catabolism pathway and several organic acids were measured during the response to nitrogen starvation in the cyanobacterium Synechocystis sp. PCC 6803 wild type and in an nblD deletion strain. A characteristic signature of metabolite pool composition was identified, which shows that NblD-mediated phycobilisome degradation is required to maintain the cellular amino acid and organic acid pools during nitrogen starvation. Specific deviations from the wild type suggest wider-reaching effects that also affect such processes as redox homeostasis via glutathione and tetrapyrrole biosynthesis, both of which are linked to the strongly decreased glutamate pool, and transcriptional reprogramming via an enhanced concentration of 2-oxoglutarate, the metabolite co-regulator of the NtcA transcription factor. The essential role played by NblD in metabolic homeostasis is consistent with the widespread occurrence of NblD throughout the cyanobacterial radiation and the previously observed strong positive selection for the nblD gene under fluctuating nitrogen supply. ImportanceCyanobacteria play important roles in the global carbon and nitrogen cycles. In their natural environment, these organisms are exposed to fluctuating nutrient conditions. Nitrogen starvation induces a coordinated nitrogen-saving program that includes the breakdown of nitrogen-rich photosynthetic pigments, particularly phycobiliproteins. The small protein NblD was recently identified as an effector of phycobilisome breakdown in cyanobacteria. In this study, we demonstrate that the NblD-mediated degradation of phycobiliproteins is needed to sustain cellular pools of soluble amino acids and other crucial metabolites. The essential role played by NblD in metabolic homeostasis explains why genes encoding this small protein are conserved in almost all members of cyanobacterial radiation.

microbiology↗

bioRxiv2020

Discovery of a novel small protein factor involved in the coordinated degradation of phycobilisomes in cyanobacteria

Phycobilisomes are the major pigment-protein antenna complexes that perform photosynthetic light harvesting in cyanobacteria, rhodophyte and glaucophyte algae. Up to 50% of the cellular nitrogen can be stored in their giant structures. Accordingly, upon nitrogen depletion, phycobilisomes are rapidly degraded. This degradation is tightly coordinated, follows a genetic program and involves small proteins serving as proteolysis adaptors. Here, we describe the role of NblD, a novel factor in this process in cyanobacteria. NblD is a cysteine-rich, 66-amino acid small protein that becomes rapidly induced upon nitrogen starvation. Deletion of the nblD gene in the cyanobacterium Synechocystis prevents the degradation of phycobilisomes, leading to a nonbleaching (nbl) phenotype. Competition experiments provided direct evidence for the physiological importance of NblD. Complementation by a plasmid-localized gene copy fully restored the phenotype of the wild type. Overexpression of NblD under nitrogen-replete conditions showed no effect, in contrast to the unrelated proteolysis adaptors NblA1 and NblA2, which can trigger phycobilisome degradation ectopically. Transcriptome analysis revealed that nitrogen starvation correctly induces nblA1/2 transcription in the {Delta}nblD strain implying that NblD does not act as a transcriptional (co-)regulator. However, fractionation and coimmunoprecipitation experiments indicated the presence of NblD in the phycobilisome fraction and identified the {beta}-phycocyanin subunit as its target. These data add NblD as a new factor to the genetically programmed response to nitrogen starvation and demonstrate that it plays a crucial role in the coordinated dismantling of phycobilisomes when nitrogen becomes limiting. Significance StatementDuring genome analysis, genes encoding small proteins are frequently neglected. Accordingly, small proteins have remained underinvestigated in all domains of life. Based on a previous systematic search for such genes, we present the functional analysis of the small protein NblD in a photosynthetic cyanobacterium. We show that NblD plays a crucial role during the coordinated dismantling of phycobilisome light-harvesting complexes. This disassembly is triggered when the cells run low in nitrogen, a condition that frequently occurs in nature. Similar to the NblA proteins that label phycobiliproteins for proteolysis, NblD binds to phycocyanin polypeptides but has a different function. The results show that, even in a well-investigated process, crucial new players can be discovered if small proteins are taken into consideration.

microbiology↗