Increasing activity of the GS-GOGAT cycle highlights the compensation of N-assimilation in the absence of nitrogen and its metabolic effects in cyanobacteria

Comparative studies of proteome analysis, photosynthetic activity and fatty acid biosynthesis of the glnA- and glsF-overexpressing strains and the wild-type (WT) were carried out to elucidate the possible role(s) of the two enzymes, glutamine synthetase (GS) and glutamate synthase (GOGAT) in the GS-GOGAT cycle under nitrogen starvation. The evidence indicated the role of GS and GOGAT in the regulatory mechanism of carbon (C-) and nitrogen (N-) assimilation and the induction of photosynthesis and lipid/fatty acid biosynthesis pathway under N-stress. The fatty acid profile of the GS-overexpressing strain (WT + GlnA) under the stress showed a drastic increase in the levels of C18:1^Δ9 and C18:1^Δ11, whereas only C18:1^Δ9 was significantly induced in the GOGAT-overexpressing strain (WT + GlsF). Moreover, the proteome analysis and oxygen evolution data showed that the extra-GS activity in the mutant (MT) cells led to the increasing of proteins involved in photosynthesis as well as the rate of oxygen evolution in the first 24 h after nitrogen depletion, suggesting its role in promoting conversion of light energy to chemical energy under the stress.