The hetZ gene indirectly regulates heterocyst development at the level of pattern formation in Anabaena sp. strain PCC 7120

Abstract

Multicellular development requires the careful orchestration of gene expression to correctly create and position specialized cells. In the filamentous cyanobacterium Anabaena sp. strain PCC 7120, nitrogen-fixing heterocysts are differentiated from vegetative cells in a reproducibly periodic and physiologically relevant pattern. While many genetic factors required for heterocyst development have been identified, the role of HetZ has remained unclear. Here, we present evidence to clarify the requirement of hetZ for heterocyst production and support a model where HetZ functions in the patterning stage of differentiation. We show that a clean, nonpolar deletion of hetZ fails to express the developmental genes hetR, patS, hetP and hetZ correctly and fails to produce heterocysts. Complementation and overexpression of hetZ in a hetP mutant revealed that hetZ was incapable of bypassing hetP, suggesting that it acts upstream of hetP. Complementation and overexpression of hetZ in a hetR mutant, however, demonstrated bypass of hetR, suggesting that it acts downstream of hetR and is capable of bypassing the need for hetR for differentiation irrespective of nitrogen status. Finally, protein-protein interactions were observed between HetZ and HetR, Alr2902 and HetZ itself. Collectively, this work suggests a regulatory role for HetZ in the patterning phase of cellular differentiation in Anabaena.

Expression from DIF1-motif promoters of hetR and patS is dependent on HetZ and modulated by PatU3 during heterocyst differentiation

“…Videau et al showed that deletion of hetZ blocked the induced expression of hetR, hetP and patS [30]. A similar effect of hetZ mutation on the expression of patS had been shown in our previous study [27].…”

“…The expression of patS was shown to be dependent on hetZ rather than hetP (S1 Fig). Consistently, patS was upregulated in a Δ hetP mutant but not in a Δ hetZ mutant [30].…”

“…hetZ and hetP functionally overlap with each other, and co-expression of these two genes was shown to restore heterocyst formation in hetR -null mutants [29]. In a different substrain of Anabaena 7120, expression of hetZ alone restored heterocyst formation in a hetR -deletion mutant [30]. The variable requirement for hetP expression may depend on differences in genetic backgrounds of substrains [31].…”

“…Most importantly, which of HetR, HetZ and HetP is required for the regulation of DIF1-motif promoters? In Anabaena 7120, deletion of hetZ blocked the induced expression of hetR , hetP and patS , whereas hetP showed no effects on these genes [30]. This result excluded HetP, but did not establish either HetR or HetZ, as the factor required for the induced expression of hetR and patS .…”

cyAbrB Transcriptional Regulators as Safety Devices To Inhibit Heterocyst Differentiation in Anabaena sp. Strain PCC 7120

“…hetR encodes a master regulator of heterocyst differentiation, and its overexpression causes heterocyst formation in the presence of nitrogen sources (13). Both hetP and hetZ are directly upregulated by HetR, and ectopic expression of either gene leads to heterocyst formation in the presence of nitrogen sources (4,(36)(37)(38)(39)(40). hepA encodes a component of an ABC transporter required for the construction of the heterocyst exopolysaccharide layer (41), which is the first step during morphological differentiation (4).…”