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Chapter 6 : Heterocyst Development and Pattern Formation

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Abstract:

Most of the information about heterocyst development to date is based on the study of three species of heterocyst-forming filamentous cyanobacteria: (also ) sp. strain PCC 7120, ATCC 29413, and ATCC 29133. This chapter focuses on those genes involved in signaling and regulation. HetR plays a central role in heterocyst development and pattern formation. In bacteria, calcium ions play important roles in various cellular processes such as pathogenesis, sporulation in , chemotaxis in , and heterocyst development in cyanobacteria. PatA may influence heterocyst development by attenuating the negative effects of the main inhibitory signals of heterocyst pattern formation, PatS and HetN. Late stages of heterocyst development are characterized by structural changes that include the deposition of three cell layers: an outermost fibrous layer, an envelope polysaccharide layer, and an innermost glycolipid layer. During nitrogen fixation, nitrogenase reduces atmospheric nitrogen to ammonia, which is then assimilated into amino acids. A recent epistasis analysis of four genes involved in pattern formation in Strain PCC 7120 suggests that PatA has two distinct activities, to promote differentiation as well as to attenuate the negative effects of PatS and HetN on differentiation. Some genes that are required for heterocyst development are also involved in akinete formation, such as hetR and hepA. In the absence of heterocysts, the akinetes seem to form at random positions along the filament, whereas the presence of heterocysts influences akinete positioning, implying the existence of cell-to-cell communication.

Citation: Aldea M, Kumar K, Golden J. 2008. Heterocyst Development and Pattern Formation, p 75-90. In Winans S, Bassler B (ed), Chemical Communication among Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555815578.ch6

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Anabaena variabilis
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Cellular Processes
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Nitrogen Fixation
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Figures

Image of FIGURE 1
FIGURE 1

Wild-type and mutant filaments of PCC 7120. (A) Wild-type filaments grown on medium containing nitrate consist of vegetative cells only. (B) Wild-type filaments grown on medium without a source of combined nitrogen have a pattern of single heterocysts spaced along filaments. (C) A deletion mutant (strain AMC451) grown on medium without a source of combined nitrogen has an Mch phenotype. (D) Green fluorescent protein fluorescence of a P reporter strain (AMC484) grown on medium without a source of combined nitrogen is localized to heterocysts (lower panel). Arrowheads indicate heterocysts.

Citation: Aldea M, Kumar K, Golden J. 2008. Heterocyst Development and Pattern Formation, p 75-90. In Winans S, Bassler B (ed), Chemical Communication among Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555815578.ch6
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Image of FIGURE 2
FIGURE 2

Signal and metabolite flow between heterocysts and vegetative cells. Wild-type filament with a normal pattern of heterocysts (marked by the arrowheads) grown on medium without a source of combined nitrogen. Vegetative cells provide carbohydrates produced by photosynthesis to heterocysts, which in turn provide fixed nitrogen as amino acids to the reproductive vegetative cells. Heterocyst pattern is controlled by PatS- and HetN-dependent signals and by the supply of nitrogen from heterocysts. The transfer and movement of small molecules along filaments is thought to occur via the periplasm ( ). Horizontal arrows indicate the apparent effective range of each signal or metabolite.

Citation: Aldea M, Kumar K, Golden J. 2008. Heterocyst Development and Pattern Formation, p 75-90. In Winans S, Bassler B (ed), Chemical Communication among Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555815578.ch6
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Image of FIGURE 3
FIGURE 3

Model showing the influence of cell-to-cell signals on heterocyst development and pattern formation. Nitrogen starvation results in accumulation of 2-oxoglutarate (2-OG), which leads to increased NtcA activity. Initiation of heterocyst development is controlled mainly by NtcA and HetR, which are positively autoregulated and mutually dependent on each other for upregulation; HetR also has autoproteolytic activity. The DNA-binding protein NrrA serves as a regulatory link between NtcA and HetR. It is directly activated by NtcA and then causes the initial induction of ;further induction of relies on autoregulatory positive feedback. HetR is considered the master regulator of heterocyst development. NtcA and HetR collaborate to reduce the levels of CcbP in the differentiating cell, the former by directly inhibiting transcription and the latter by proteolysis, resulting in release of free calcium. NtcA is required for early as well as late stages of heterocyst development. HetF positively influences heterocyst development by an unknown mechanism. Three factors produced by the differentiating cell are proposed to influence heterocyst development and pattern formation by acting as cell-to-cell signals (enclosed in boxes). Fixed nitrogen originating from the differentiating cells and mature heterocysts in the form of amino acids negatively influence differentiation of neighboring cells and establish the ultimate spacing between heterocysts. Production of PatS is directly activated by HetR in differentiating cells, and it is thought that PatS, or a posttranslationally processed peptide, is secreted and then enters neighboring cells where it inhibits HetR activity to block differentiation. HetN is also thought to generate an inhibitory signal from mature heterocysts that inhibits differentiation of adjacent cells, possibly by interfering with HetR activity. PatA may partially relieve the inhibitory effect of the HetN- and PatS-dependent signals. The dashed arrow indicates putative posttranslational processing of PatS, and the large arrows represent other developmental steps that take place between the activation of HetR and completion of the differentiation process.

Citation: Aldea M, Kumar K, Golden J. 2008. Heterocyst Development and Pattern Formation, p 75-90. In Winans S, Bassler B (ed), Chemical Communication among Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555815578.ch6
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