The cyanobacteria share with plants the capacity to perform oxygenic photosynthesis by using light and water for the reduction of CO2. Endowed with capacities of acclimation to a wide range of environmental conditions, including extreme ones, they colonize most aquatic and terrestrial ecosystems. Some of them produce hepatotoxins or neurotoxins harmful to animals and Man. Research work in the Unit is focussed on the development and valorisation of the collection of cyanobacteria (PCC), in particular strains potentially toxic, and on the study of the molecular mechanisms controlling the acclimation of these micro-organisms to changes in environmental parameters.

Our Unit possesses the largest collection of cyanobacteria in the world by the number of strains in pure cultures and by the diversity of their geographical origin and properties.

The PCC has several missions:

• Preservation of biological resources (more than 740 pure strains);

• Consulting and sale of strains to research laboratories and industries all over the world; the number of strain requests in 2001 has been of more than 250;

• Isolation, purification and identification of strains by using a polyphasic approach; in 2001, new strains belonging to the genera Merismopedia, Coelosphaerium, Microcystis, Oscillatoria and Planktothrix entered the PCC;

• Search for and characterisation of bio-active molecules of pharmaceutical interests, including toxins; a transverse research programme (PTR), entitled "Neurotoxins and related compounds", coordinated by M. Herdman, is ongoing (collaborations with H-O Nghiem from the Molecular Neurobiology unit, B. Baron from the Eukaryotic Gene Expression Unit and O. Barzu from the Laboratory of Structural Chemistry of Macromolecules); a collaboration has been also established with V. Bultel from the "Laboratoire de Chimie des Substances Naturelles du Museum National d'Histoire Naturelle, Paris.

• Development of molecular tools for the detection, identification and survey of cyanobacteria in laboratory cultures or in the environment; new tools have been designed for in situ hybridization and genotyping of potentially toxic strains.

In collaboration with J.M. Delattre (Department of Waters and Environment, IP-Lille), expert tests have been set up for the identification and the determination of hepatotoxins in samples from surface waters.

Several databases are in constant evolution:

• A catalogue is available on the Web, via the server of the Institut Pasteur (http://www.pasteur.fr/recherche/banques/PCC/);

• The database CYANOBANK houses the properties of 612 strains from the PCC and 610 photomicrographs;

• The database of 16S rRNA sequences, representative of the biodiversity of cyanobacteria, consists of more than 630 aligned sequences, that of bacterial 16S RNA houses 13000 sequences; a database of 23 rRNA sequences, started recently, includes 19 complete sequences;

• The database of ITS consists of 138 aligned sequences and 344 entries of their number and size; the development of this database will permit a rapid identification of the strains and will be a very valuable tool for quality control tests.

A second edition of the Bergey's Manual of Systematic Bacteriology has been published in 2001 in which the descriptions of the cyanobacterial strains are largely based on studies performed by our team on strains from the PCC.

Light and nutrients act in a number of circuits regulating cyanobacterial metabolism, and may, in some strains belonging to the genus Calothrix, control the differentiation of cells specialized for nitrogen fixation (heterocysts), and that of motile minifilaments (hormogonia), and/or regulate changes in pigment composition (complementary chromatic adaptation).

Two cyanobacterial phytochromes, CphA and CphB, have been genetically characterized in Calothrix PCC 7601. These light-regulated histidine kinases act at an early step in the transmission of the red/far red light signal by a mechanism of phosphorylation/dephosphorylation of the response regulators RcpA and RcpB. In contrast to CphA that carries a covalently bound chromophore, CphB carries a non-covalently bound one. No cross talk has been detected between CphA-RcpA and CphB-RcpB, indicating that these couples are specific. Based on studies using a bacterial two-hybrid system, the RcpA protein forms homodimers. The 3D-structure of both RcpA and RcpB has been established. The biochemical and spectral properties of different mutants are being studied to elucidate the structure-function relationships of these four proteins (collaborations with W. Gärtner, Max Plank Institute, Mülheim der Rürh, Germany, and T. Hübschmann and T. Börner, Humboldt University, Germany, and with D. Ladant and G. Karimova, Unité de Biochimie Cellulaire, IP).

The primary structure of the PII (GlnB) protein is very conserved among prokaryotes and plants, its function, however, differs depending on organisms. In order to get deeper insights into the role of this protein in cell metabolism, different cyanobacteria are being studied. In the strains Synechococcus PCC 7942, obligate photoautotroph, and Synechocystis PCC 6803, facultative heterotroph, the phosphorylated form of PII inhibits the uptake of nitrate/nitrite ions. This inhibition is relieved when PII is liganded to 2-oxoglutarate and phosphorylated, i.e. in the presence of high carbon and low nitrogen concentrations. In Synechocystis, this modified form of PII inhibits the high affinity transport system for bicarbonate ions. Finally, the phosphorylation level of PII in both Synechococcus and Synechocystis depends on the intracellular redox potential and, consequently, the photosynthetic activity of the cell (collaboration with S. Bédu and R. Jeanjean, LCB, Marseille). This regulatory process allows cells to maintain a proper N/C balance in response to changes in light and nutrients (nitrogen and carbon) in the environment.

In the frame of a European Programme (PROMOLEC "Prochlorococcus molecular ecology", programme MAAST III, 1998-2001), some characteristics of Prochlorococcus marinus, a cyanobacterium extremely abundant in oligotrophic tropical and subtropical oceans, have been studied. This strain has the smallest cell size (diameter 0.5-0.6 mm), the smallest genome (1,67 Mb), the lowest G+C content (31 %) among cyanobacteria, and possesses atypical light harvesting antennae that consist of protein complexes containing divinyl-chlorophylls a and b but no phycobiliproteins.

P. marinus has a cyanobacterial-type PII that remains unphosphorylated, although its primary and quaternary structures are well-conserved (collaborations with S. Loiseaux-De Goër, CNRS, Roscoff, and A. Blondel, Unité de Biochimie cellulaire, IP). We have shown that P. marinus lacks the genetic information required for the assimilation of nitrate and nitrite ions. In spite of its lack of post-translational phosphorylation, the PII protein, however, might control the assimilation of the bicarbonate ions in response to changes in the intracellular concentration of 2-oxoglutarate (collaboration with S. Bédu, LCB-CNRS, Marseille).

In silico analyses of the sequence of the Synechocystis PCC 6803 genome revealed the presence of repeated motifs whose functions remain to be elucidated (collaboration with M. F. Sago, Service d'Informatique Scientifique, IP, and H. Geiselmann, Université J. Fourier, Grenoble). CyanoList, a genome browser for Synechocystis PCC6803, with the same Web server structure as SubtiList, has been created (collaboration with I. Moszer, Unité de Génétique des Génomes Bactériens).

Our team participates in a project on the sequencing of the genome of P. marinus SS120 (collaboration with the Génoscope, Evry).

In situ hybridizations (FISH) of potentially hepatotoxic cyanobacteria belonging to the genus Microcystis (green) and of eukaryotic organisms (amoebae; red) from a water sample. Detection with two different fluorochromes coupled to tyramide and observation under confocal laser scanning microscopy (W. Shönhuber and M. Herdman).

LEFEBVRE Jacqueline (jlefebvr@pasteur.fr)

RIPPKA-HERDMAN Rosmarie, IP (CR, rrippka@pasteur.fr)

WU Tianfu, Postdoctoral Researcher

LAURENT Thierry, IP (TL, tlaurent@pasteur.fr)