Transcriptional regulation of development in heterocyst-forming cyanobacteria

Abstract

Filamentous, heterocyst-forming cyanobacteria are among the simplest multicellular systems in Nature. In the absence of combined nitrogen, the filaments consist of vegetative cells that fix CO through oxygenic photosynthesis and micro-oxic heterocysts specialized for the fixation of N in a proportion of about 10 to 1. The development of a heterocyst-containing filament involves differentiation of vegetative cells into heterocysts in a process that requires a distinct gene expression program. Two transcription factors are strictly required, NtcA and HetR. The CRP-family protein NtcA directly activates the expression of multiple genes during heterocyst differentiation - in some cases assisted by coactivators including HetR - and in mature heterocysts, whereas HetR is needed to build high NtcA levels in differentiating heterocysts and directly activates some particular genes. A few other regulators of gene expression participate at specific differentiation steps, and a specific transcription factor, CnfR, activates nif gene expression under the micro-oxic conditions of the heterocyst.

Elements of the heterocyst‐specific transcriptome unravelled by co‐expression analysis in Nostoc sp. PCC 7120

“…C). Consistent with their known sequential roles in the differentiation process (reviewed in Flores et al ., ), the hep (heterocyst polysaccharide) genomic islands appeared in the E‐DIF cluster, while hgl (heterocyst glycolipids) and nif genes appeared in the L‐DIF cluster (Fig. B).…”

Genetic responses to carbon and nitrogen availability in Anabaena

“…High levels of expression of the operon are observed only if nitrate or nitrite (which can be considered physiological inducers) is available. This requirement for nitrate (nitrite) is, however, not observed in nirA or nirB mutants, which express the operon at high levels in the absence of the inducer (Frías and Flores, 2010), suggesting a role of the NirA/NirB complex as a direct or indirect repressor of the operon (Fig. 2).…”

“…The ntcB gene is expressed also from a Class II NtcAactivated promoter (Frías et al, 2000). Further, the products of two genes, nirB and narM, are needed to attain high levels of activity of nitrite reductase and nitrate reductase respectively, and NirA-NirB and NarB-NarM proteinprotein interactions have been found to take place (Frías and Flores, 2010;. High levels of expression of the operon are observed only if nitrate or nitrite (which can be considered physiological inducers) is available.…”

“…Heterocyst differentiation requires the action of the products of multiple structural and regulatory genes, which have been identified through the isolation of mutants impaired in the differentiation process and, more recently, by global gene expression studies. Many of the genes needed for heterocyst differentiation are activated at specific stages during the process, only in the differentiating cells (see Herrero et al, 2013;Flores et al, 2018).…”

Proteome Analysis of Enriched Heterocysts from Two Hydrogenase Mutants from Anabaena sp. PCC 7120

“…Heterocysts often make use of additional cell wall envelopes to generate microaerobic environments inside the cell to favor the N 2 fixation processes . Different proteins are involved in the synthesis of the polysaccharide (Hep) and glycolipid (Hgl) layer and have been identified in Anabaena and are regulated by ntcA and hetR . Several genes, hepA , hepB , hepC which are responsible for heterocyst differentiation and formation of the polysaccharide layer, were detected only in the wild type .…”