The development of a pattern of differentiated cell types from a group of equivalent cells is a fundamental paradigm in biology. The hallmark of differentiation is the creation of self-sustaining patterns of gene regulation and protein activity.
Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium that can be induced to differentiate a periodic pattern of nitrogen-fixing heterocysts from a chain of undifferentiated vegetative cells. Heterocysts occur, on average, at 10 cell intervals, are terminally differentiated, and differ from vegetative cells morphologically, metabolically, and genetically.
They allow the spatial separation of the two incompatible processes of photosynthesis and nitrogen fixation. As an example of division of labor between cells type, heterocysts provide vegetative cells with fixed nitrogen and receive fixed carbon from vegetative cells.
Patterning of differentiation appears to be dependent on the interactions between several proteins, some of which are indicated above in a schematic of the stages of differentiation. The first, HetR, is part of a regulatory circuit that shares the properties of biological switches, which turn graded input signals into a binary output: when the switch is “off”, the cell remains undifferentiated, but when the switch is “turned on”, the differentiation process begins and eventually becomes irreversible and self-sustaining. HetR acts to promote differentiation and is both necessary and sufficient to induce differentiation. PatS is a protein that prevents differentiation, and is responsible for determining the de novo patterning of heterocysts on a filament. HetN produces a signal involved in stabilization and maintenance of the pattern once it has formed.
The relative positions of cells is conveyed by concentration gradients of PatS and/or HetN extending from source cells. PatS and HetN prevent the activity of HetR and cause its decay as can be seen in the micrograph above where fluorescence from HetR-GFP is reduced next to heterocysts, which are sources of HetN and PatS. The phenomenon of “lateral inhibition” of differentiation has been proposed to govern patterning in many developmental systems.
Lateral inhibition in Anabaena is most easily conceptualized when considering the placement of a new heterocyst between two existing heterocysts after the vegetative cells between them have divided.