Abstract
Nutrient balance is important for photosynthetic growth and biomass production in microalgae. Here, we investigated and compared metabolic responses of amino acid pools to nitrogen and sulfur starvation in a unicellular model cyanobacterium, Synechocystis sp. PCC 6803, and its mutant nblA1/A2. It is known that NblA1/A2-dependent and -independent breakdown of abundant photosynthetic phycobiliproteins and other cellular proteins supply nutrients to the organism. However, the contribution of the NblA1/A2-dependent nutrient supply to amino acid pool homeostasis has not been studied. Our study demonstrates that changes in the pool size of many amino acids during nitrogen starvation can be categorized as NblA1/A2-dependent (Gln, Glu, glutathione, Gly, Ile, Leu, Met, Phe, Pro, Ser, Thr, Tyr and Val) and NblA1/A2-independent (Ala, Asn, Lys, and Trp). We also report unique changes in amino acid pool sizes during sulfur starvation in wild type and the mutant and found a generally marked increase in the Lys pool in cyanobacteria during nutrient starvation. In conclusion, the NblA1/A2-dependent protein turnover contributes to the maintenance of many amino acid pools during nitrogen starvation.
Highlights
Nutrient balance is critical for the phototrophic growth of microalgae, because proteins, nucleic acids, carbohydrates, lipids, and pigments must be supplied in a ratio suitable for actively growing cells
To quickly and quantitatively extract free amino acids from Synechocystis cells, cells were collected with a filter and disrupted with zirconia beads, and free amino acids were extracted with methanol
NblA1/A2-Dependent and -Independent Homeostasis of Amino Acids In Figure 1, we summarized the NblA1/A2-dependent and -independent amino acid homeostasis during nitrogen and sulfur starvation in the model cyanobacterium Synechocystis sp
Summary
Nutrient balance is critical for the phototrophic growth of microalgae, because proteins, nucleic acids, carbohydrates, lipids, and pigments must be supplied in a ratio suitable for actively growing cells. A unique NblA-dependent mechanism is induced to degrade certain phycobiliproteins, which are major components of the phycobilisome, the supramolecular photosynthetic antenna that harvests light for photosynthesis in these organisms. They are soluble proteins attached to the thylakoid surface and may constitute up to ~50% of total soluble proteins in many cyanobacterial cells [9]. Because the small protein NblA directly binds to certain phycobiliproteins, its binding may trigger proteolytic breakdown of the target proteins [13] It is, still unknown to what extent NblA-dependent protein degradation contributes to the amino acid supply during nitrogen starvation [14,15,16,17]. To investigate the contribution of amino acid recycling in Synechocystis, we analyzed the metabolic responses of amino acid pools in Synechocystis and its mutant nblA1/A2 to nitrogen and sulfur starvation
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