Fig 1. Basic regulatory network leading to heterocyst differentiation.

In response to combined nitrogen deprivation, the NtcA protein is activated, leading to activation of expression of HetR in some cells. NtcA and HetR undergo mutual amplification, resulting in increased levels of the two regulators. In addition, HetR positively regulates its own production. During the early stages of differentiation, HetR induces expression of PatS in cells that can potentially form heterocysts (yellow). A PatS-derived peptide signal is thought to diffuse to neighbouring cells (grey gradient), where it interferes with the DNA-binding activity of HetR, causing its degradation and creating HetR gradients along filaments. At late stages (dashed arrow), HetN is produced in heterocysts (orange), and a HetN-derived signal is conveyed to neighbouring cells (grey gradient), where it inhibits HetR function and heterocyst formation. During the differentiation process, cells lose phycobilisomes and their autofluorescence declines (different shades of green). Phycobilisomes are restored when nitrogen compounds produced by the heterocysts reach the vegetative cells. For details, see Introduction. The dynamical variables of our model, the activator HetR and its inhibitors PatS and HetN, have been emphasized in red. Adapted with modifications from [6].