Elucidating a Mechanism of Growth Cessation in Heterocysts of Anabaena sp. PC7120

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Anabaena sp. strain PCC 7120 is a filamentous, multicellular cyanobacterium that allows for the study of developmental factors that lead to the patterned differentiation of cell types. When given a source of fixed nitrogen, Anabaena grows in long strands of identical vegetative cells, all of which carry out photosynthesis to capture energy and fix carbon for the organism. However, under nitrogen-deplete conditions, approximately every tenth vegetative cell of a strand will terminally differentiate into a heterocyst, a non-dividing cell type with the ability to fix atmospheric nitrogen. This cell-type specific cessation of division indicates that there is a regulatory link between binary fission and heterocyst differentiation. In Anabaena, the location of the division plane for binary fission is determined by the interaction of MinC, MinD, MinE, and FtsZ. It is unknown how cell division is inhibited in heterocysts, but it has been shown that heterocysts lack polymerized FtsZ rings (Z-rings) normally responsible for for cytokinesis. To begin to understand FtsZ loss in heterocysts, mutant strains of Anabaena that displayed abnormal heterocyst morphology were screened for the presence of Z-rings. In mutant strains of interest, a copy of ftsZ, translationally fused with a yellow fluorescent protein (yfp), was introduced into the genome, and the cells were visualized by fluorescence microscopy. The results suggest that hetP may be responsible for inhibiting Z-ring formation in heterocysts, thus preventing division. Through a bacterial two-hybrid system and β-galactosidase assay, the interactions of HetP, Min proteins, and FtsZ have been qualitatively and quantitatively assessed. The results suggest an indirect interaction between HetP and cell division proteins as a possible mechanism for the cessation of growth in the heterocyst cell type of Anabaena. Forward genetic screening has lead to the establishment of a potential downstream target of HetP.



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