| The formation of a pattern of differentiated cells from a group of seemingly equivalent, undifferentiated cells is a central paradigm of developmental biology. Several species of filamentous cyanobacteria differentiate nitrogen-fixing heterocysts at regular intervals along unbranched filaments to form a periodic pattern of two distinct cell types. HetR is an activator of heterocyst differentiation that is positively auto-regulated at the level of transcription. PatS and HetN are inhibitors of differentiation that depend on HetR for their production and contain the pentapeptide RGSGR, which inhibits heterocyst formation in vivo and HetR-DNA-binding activity in vitro. In the patterning of heterocysts as in many developmental systems it has been proposed that activators and inhibitors of differentiation diffuse from source cells to form concentration gradients that in turn mediate patterning, but direct visualization of concentration gradients of activators and inhibitors has been difficult. Here we show that the periodic pattern of heterocysts produced by cyanobacteria rely on two inhibitors of heterocyst differentiation, PatS and HetN, in a manner consistent with the predictions of the activator-inhibitor model. Concentration gradients of the activator, HetR, were observed adjacent to heterocysts, the natural source of PatS and HetN, as well as adjacent to vegetative cells that were manipulated to over-express a gene encoding either of the inhibitors. Gradients of HetR relied on post-translational decay of HetR. Deletion of both patS and hetN genes prevented the formation of gradients of HetR, and a derivative of the inhibitors was shown to promote decay of HetR in a concentration-dependent manner. At least two separate pathways regulated the decay of HetR, one involving inhibitor-dependence, and the other requiring HetF-, and possibly HetR-protease activity. Our results provide strong support for application of the activator-inhibitor model to heterocyst patterning and, more generally, the formation of periodic patterns in biological systems. |
