Cross-talk between iron and nitrogen regulatory networks in anabaena (Nostoc) sp. PCC 7120: identification of overlapping genes in FurA and NtcA regulons.

Nitrogen signalling in cyanobacteria involves a complex network in which the availability of iron plays an important role. In the nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120, iron uptake is controlled by FurA, while NtcA is the master regulator of nitrogen metabolism and shows a mutual dependence with HetR in the first steps of heterocyst development. Expression of FurA is modulated by NtcA and it is enhanced in a hetR(-) background. Iron starvation in cells grown in the presence of combined nitrogen causes a moderate increase in the transcription of glnA that is more evident in a ntcA(-) background. Those results evidence a tight link between the reserves of iron and nitrogen metabolism that leads us to search for target genes potentially co-regulated by FurA and NtcA. Using a bioinformatic approach we have found a significant number of NtcA-regulated genes exhibiting iron boxes in their upstream regions. Our computational predictions have been validated using electrophoretic mobility shift assay (EMSA) analysis. These candidates for dual regulation are involved in different functions such as photosynthesis (i.e. psaL, petH, rbcL, isiA), heterocyst differentiation (i.e. xisA, hanA, prpJ, nifH), transcriptional regulation (several alternative sigma factors) or redox balance (i.e. trxA, ftrC, gor). The identification of common elements overlapping the NtcA and FurA regulons allows us to establish a previously unrecognized transcriptional regulatory connection between iron homeostasis, redox control and nitrogen metabolism.