Preventing Accidental Heterocyst Development in Cyanobacteria

Abstract

The filamentous cyanobacterium Anabaena can form heterocysts specialized in N₂ fixation, mostly through a cascade of transcriptional activation in response to the nitrogen starvation signal 2-oxoglutarate. It is reported now that a transcription repressor, CalA, acts as a safety device to prevent heterocyst development under certain conditions where the 2-oxoglutarate level may touch the threshold to trigger unnecessary initiation of heterocyst development. Such a control may increase the fitness of Anabaena under a constantly changing environment.

Keywords: Anabaena; cell differentiation; metabolic balance; nitrogen fixation; transcriptional regulation.

FIG 1

Mechanism of heterocyst development in Anabaena. (A) Fluorescent image of Anabaena filaments grown under diazotrophic conditions, with the presence of heterocysts (indicated by arrowheads) (for details of fluorescence labeling, see reference 31). (B) Steps of heterocyst development. Only major steps most closely related to the topic are depicted. Anabaena can use combined nitrogen sources, such as nitrate or ammonium, or N₂ under combined nitrogen deprivation. In the latter case, 2-oxoglutarate (2-OG), as a carbon skeleton for ammonium assimilation, accumulates and activates the transcription factor NtcA. HetR is also a transcriptional activator and a master regulator specific for heterocyst development. Inhibitory signals derived from PatS and PatX act on HetR to determine the heterocyst pattern along the filaments. hetP and hetZ are two direct targets of HetR, involved in the commitment step, making heterocyst development irreversible. At the later stages, morphogenesis takes place with the formation of the polysaccharide layer (HEP layer) and the glycolipid layer (HGL layer) at the heterocyst cell wall. HepA regulates the formation of the HEP layer. CalA negatively controls hetP and hepA. trpE is required for tryptophan synthesis from glutamine, and it affects the 2-OG pool and thus also heterocyst development. Dotted lines represent steps that need further experimental confirmation. The binding of HetR on the hepA promoter is weak in vitro and thus may not be significant in vivo (indicated by a question mark).