Glutamine synthetase from the marine cyanobacteria Prochlorococcus spp.: characterization, phylogeny and response to nutrient limitation

Abstract

The regulation of glutamine synthetase (EC 6.3.1.2) from Prochlorococcus was previously shown to exhibit unusual features: it is not upregulated by nitrogen starvation and it is not inactivated by darkness (El Alaoui et al. (2001) Appl Environ Microbiol 67: 2202-2207). These are probably caused by adaptations to oligotrophic environments, as confirmed in this work by the marked decrease in the enzymatic activity when cultures were subjected to iron or phosphorus starvation. In order to further understand the adaptive features of ammonium assimilation in this cyanobacterium, glutamine synthetase was purified from two Prochlorococcus strains: PCC 9511 (high-light adapted) and SS120 (low-light adapted). We obtained approximately 100-fold purified samples of glutamine synthetase electrophoretically homogeneous, with a yield of approximately 30%. The estimated molecular mass of the subunits was roughly the same for both strains: 48.3 kDa. The apparent Km constants for the biosynthetic activity were 0.30 mM for ammonium, 1.29 mM for glutamate and 1.35 mM for ATP; the optimum pH was 8.0. Optimal temperature was surprisingly high (55 degrees C). Phylogenetic analysis of glnA from three Prochlorococcus strains (MED4, MIT9313 and SS120) showed they group closely with marine Synechococcus isolates, in good agreement with other studies based on 16 S RNA sequences. All of our results suggest that the structure and kinetics of glutamine synthetase in Prochlorococcus have not been significantly modified during the evolution within the cyanobacterial radiation, in sharp contrast with its regulatory properties.

Shotgun proteomic analysis of the unicellular alga Ostreococcus tauri

“…The downregulation of proteins involved in nitrogen assimilation under lower nitrogen conditions although counter-intuitive has also been observed previously in Prochlorococus a marine photosynthetic prokaryote [42]. El Alaoui et al 2003 [42] present an elegant explanation regarding this behavior and this atypical response could be associated to an adaptation by both Ostreococcus and Prochlorococcus for surviving under oligotrophic conditions (i.e. under low nitrogen conditions in this case).…”

Streamlined Regulation and Gene Loss as Adaptive Mechanisms in Prochlorococcus for Optimized Nitrogen Utilization in Oligotrophic Environments

“…The reviewed literature on nitrogen assimilation in Prochlorococcus clearly suggests a simplification of the regulatory networks. This is reflected in different observations, such as the fairly constant concentration and activity of GS (9,10), the constitutive expression of the ammonium transporter (33), the lack of phosphorylation of the P II protein (56), and the apparent lack of nitrogen control by NtcA (33). This is in good agreement with the small number of response regulators and histidine kinases observed in the Prochlorococcus genomes (8,42,65,68).…”

“…Accordingly, the gene glnA, encoding GS, is found in all available genomes (8,10,65). Interestingly, the amino acid sequence, isoelectric point, molecular size, and kinetic parameters of GS from Prochlorococcus have been shown to be very similar to those of the enzymes from other cyanobacteria, either freshwater or marine strains (10), indicating very slight modifications of the properties of this enzyme during evolution.…”

Response of phytoplankton to nitrogen addition in the Taiwan strait upwelling region: Nitrate reductase and glutamine synthetase activities

“…However, the effects caused by environmental factors vary significantly among phytoplankton species. For example, GSA is enhanced by nitrogen starvation and inactivated by darkness in many phytoplankton species, but this does not occur in Prochlorococcus (El Alaoui et al, 2001;2003). The relationship between ammonium and GSA is complex: in Emiliania luxleyi, GSA is inhibited by the addition of ammonium, depending on the physiological status of the phytoplankton cells (Maurin and Gal, 1997).…”