The network of C/N balancing, photosynthesis and flavodiiron proteins in the cyanobacterium Anabaena sp. PCC 7120 – Towards bioengineering strategies

Abstract

The heterocyst-forming cyanobacterium Anabaena sp. PCC 7120 can perform oxygenic photosynthesis in vegetative cells and simultaneously O2-sensitive N2 fixation in heterocysts. In Paper I, deletion mutants of the photoprotective flavodiiron proteins flv1A and flv3A were characterized. The deletion of the vegetative cell-specific flv3A leads to downregulation of the heterocyst-specific Hup uptake hydrogenase gene. This leads to enhanced H2 production, suggesting a connection between photosynthetic e-sinks and N2 fixation. Moreover, Paper I demonstrates that both Flv1A and Flv3A are required under fluctuating light similar to Synechocystis. However, in contrast to Synechocystis, where Flv1 and Flv3 are mutually dependent, Flv3A can moderately catalyse the Mehler-like reaction independent from Flv1A. Still, this function of Flv3A requires the presence of Flv2 and Flv4, suggesting some level of cooperation under air-level CO2. The formation of heterocysts is a transcriptional response to cellular C/N imbalance which is mainly signalled by the central metabolite 2-OG. Paper II demonstrates that deletion of the global transcription factor PacR, an important regulator of C-metabolism, FDPs, and photosynthesis, leads to enhanced heterocyst formation after shifting the N source from NH4 + to NO3 - , most likely due to internal N shortage. This was primarily attributed to the downregulation of NO3 - uptake, which is directly regulated by PacR. Moreover, impaired regulation of photosynthetic components in the absence of PacR may have led to the slight decrease in the size of the photo-reducible Fed-pool, thereby decreasing the availability of reducing equivalents required for NO3 - reduction. Additionally, NH4 + assimilation was disrupted due to GS/GOGAT-cycle impairment. Thus, these findings indicate that PacR functions as a regulator of N metabolism and C/N balancing and confirm the regulation of flavodiiron proteins.