Sophie Laurent

Heterocyst differentiation and pattern formation in cyanobacteria: a chorus of signals

Heterocyst differentiation in filamentous cyanobacteria provides an excellent prokaryotic model for studying multicellular behaviour and pattern formation. In Anabaena sp. strain PCC 7120, for example, 5–10% of the cells along each filament are induced, when deprived of combined nitrogen, to differentiate into heterocysts. Heterocysts are specialized in the fixation of N2 under oxic conditions and are semi‐regularly spaced among vegetative cells. This developmental programme leads to spatial separation of oxygen‐sensitive nitrogen fixation (by heterocysts) and oxygen‐producing photosynthesis (by vegetative cells). The interdependence between these two cell types ensures filament growth under conditions of combined‐nitrogen limitation. Multiple signals have recently been identified as necessary for the initiation of heterocyst differentiation, the formation of the heterocyst pattern and pattern maintenance. The Krebs cycle metabolite 2‐oxoglutarate (2‐OG) serves as a signal of nitrogen deprivation. Accumulation of a non‐metabolizable analogue of 2‐OG triggers the complex developmental process of heterocyst differentiation. Once heterocyst development has been initiated, interactions among the various components involved in heterocyst differentiation determine the developmental fate of each cell. The free calcium concentration is crucial to heterocyst differentiation. Lateral diffusion of the PatS peptide or a derivative of it from a developing cell may inhibit the differentiation of neighbouring cells. HetR, a protease showing DNA‐binding activity, is crucial to heterocyst differentiation and appears to be the central processor of various early signals involved in the developmental process. How the various signalling pathways are integrated and used to control heterocyst differentiation processes is a challenging question that still remains to be elucidated.

Heme Alkylation by Artesunic Acid and Trioxaquine DU1301, Two Antimalarial Trioxanes

The sesquiterpene Artemisinin, an antimalarial drug that is effective against multidrug‐resistant Plasmodium falciparum strains, contains a 1,2,4‐trioxane, and the endoperoxide function plays a key role in its biological activity. However, its poor solubility means that hemisynthetic derivatives, such as artesunic acid, are preferred for drugs.

Cell-type specific modification of PII is involved in the regulation of nitrogen metabolism in the cyanobacterium Anabaena PCC 7120

In the heterocystous cyanobacterium Anabaena PCC 7120, the modification state of the signalling PII protein is regulated according to the nitrogen regime of the cells, as already observed in some unicellular cyanobacteria. However, during the adaptation to diazotrophic growth conditions, PII is phosphorylated in vegetative cells while unphosphorylated in heterocysts. Isolation of mutants affected on PII modification state and analysis of their phenotypes allow us to show the implication of PII in the regulation of molecular nitrogen assimilation and more specifically, the requirement of unmodified state of PII in the formation of polar nodules of cyanophycin in heterocysts.