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Model of the relationships among some of the gene products that control heterocyst differentiation in Anabaena. Normally, the environmental signal of low fixed nitrogen is perceived by the NtcA protein, as modified by the regulator PII. NtcA activates transcription of many genes, eventually including hetR. When modified by PatA, HetR activates expression of itself and other genes leading to heterocyst differentiation. HetR action is countered by both PatS and HetN.
