Physiological Studies of Glutamine Synthetases I and III from Synechococcus sp. WH7803 Reveal Differential Regulation

Abstract

The marine picocyanobacterium Synechococcus sp. WH7803 possesses two glutamine synthetases (GSs; EC 6.3.1.2), GSI encoded by glnA and GSIII encoded by glnN. This is the first work addressing the physiological regulation of both enzymes in a marine cyanobacterial strain. The increase of GS activity upon nitrogen starvation was similar to that found in other model cyanobacteria. However, an unusual response was found when cells were grown under darkness: the GS activity was unaffected, reflecting adaptation to the environment where they thrive. On the other hand, we found that GSIII did not respond to nitrogen availability, in sharp contrast with the results observed for this enzyme in other cyanobacteria thus far studied. These features suggest that GS activities in Synechococcus sp. WH7803 represent an intermediate step in the evolution of cyanobacteria, in a process of regulatory streamlining where GSI lost the regulation by light, while GSIII lost its responsiveness to nitrogen. This is in good agreement with the phylogeny of Synechococcus sp. WH7803 in the context of the marine cyanobacterial radiation.

The Distinctive Regulation of Cyanobacterial Glutamine Synthetase

“…Recently, physiological examination of GSIII from Synechococcus sp. WH7803 revealed an unusual regulation of this enzyme as GSIII activity in this strain is highly responsive to light-dark changes while being unaffected by N availability [151]. Thus, the assumption that GSIII promotes recovery after prolonged N starvation [58] appears unlikely in Synechococcus sp.…”

“…WH7803. Given that all sequenced Prochlorococcus strains lack the glnN gene, it appears that GSIII was lost in the course of streamlining in environments with stable nutrient availability [151]. Therefore, Synechococcus sp.…”

“…More recently, analysis of Synechococcus sp. RS9917 revealed that this marine cyanobacterium possesses two GSIII enzymes and lacks GSI [151]. In contrast to the situation in Synechocystis where nitrate-supplementation causes basal GSIII levels, glnN expression is strongly induced in nitrate-grown as well as N-starved cells of Pseudanabaena 6903, while ammonium addition triggers a rigorous inhibition of enzymatic activity accompanied by steady protein levels.…”

Peer Review #2 of "Differential expression of the glucose transporter gene glcH in response to glucose and light in marine picocyanobacteria (v0.3)"

“…RNA was isolated from mL culture samples, using the TRIsure RNA Isolation Reagent (Bioline) following the method described previously (Domínguez-Martín et al 2016).…”

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“…RNA was isolated from 500 mL culture samples, using the TRIsure RNA Isolation Reagent (Bioline) following the method described previously (Domínguez-Martín et al 2016).…”