Anne-Marie Castets

The structure of cyanobacterial phycobilisomes: a model

Phycobilisomes, supramolecular complexes of water-soluble accessory pigments, serve as the major light-harvesting antennae in cyanobacteria and red algae. Regular arrays of these organelles are found on the surface of the thylakoid membranes of these organisms. In the present study, the hemi-discoidal phycobilisomes of several species of cyanobacteria were examined in thin sections of cells and by negative staining after isolation and fixation. Their fundamental structures were found to be the same. Isolated phycobilisomes possessed a triangular core assembled from three stacks of disc-shaped subunits. Each stack contained two discs which were ∼12 nm in diameter and ∼6–7 nm thick. Each of these discs was probably subdivided into halves ∼3–3.5 nm thick. Radiating from each of two sides of the triangular core were three rods ∼12 nm in diameter. Each rod consisted of stacks of 2 to 6 disc-shaped subunits ∼6 nm thick. These discs were subdivided into halves ∼3 nm thick.The average number of discs of ∼6 nm thickness forming the peripheral rods varied among the strains studied. For certain chromatically adapting strains, the average rod length was dependent upon the wavelength of light to which cells were exposed during growth. Analyses of phycobilisomes by spectroscopic techniques, polyacrylamide gel electrophoresis, and electron microscopy were compared. These analyses suggested that the triangular core was composed of allophycocyanin and that the peripheral rods contained phycocyanin and phycoerythrin (when present). A detailed model of the hemi-discoidal phycobilisome is proposed. This model can account for many aspects of phycobiliprotein assembly and energy transfer.

The Gas Vesicle Gene Cluster from Microcystis aeruginosa and DNA Rearrangements That Lead to Loss of Cell Buoyancy

Microcystis aeruginosa is a planktonic unicellular cyanobacterium often responsible for seasonal mass occurrences at the surface of freshwater environments. An abundant production of intracellular structures, the gas vesicles, provides cells with buoyancy. A 8.7-kb gene cluster that comprises twelve genes involved in gas vesicle synthesis was identified. Ten of these are organized in two operons, gvpA(I)A(II)A(III)CNJX and gvpKFG, and two, gvpV and gvpW, are individually expressed. In an attempt to elucidate the basis for the frequent occurrence of nonbuoyant mutants in laboratory cultures, four gas vesicle-deficient mutants from two strains of M. aeruginosa, PCC 7806 and PCC 9354, were isolated and characterized. Their molecular analysis unveiled DNA rearrangements due to four different insertion elements that interrupted gvpN, gvpV, or gvpW or led to the deletion of the gvpA(I)-A(III) region. While gvpA, encoding the major gas vesicle structural protein, was expressed in the gvpN, gvpV, and gvpW mutants, immunodetection revealed no corresponding GvpA protein. Moreover, the absence of a gas vesicle structure was confirmed by electron microscopy. This study brings out clues concerning the process driving loss of buoyancy in M. aeruginosa and reveals the requirement for gas vesicle synthesis of two newly described genes, gvpV and gvpW.

Characterization of two insertion sequences, IS701 and IS702, from the cyanobacterium Calothrix species PCC 7601.

We describe the characterization of two insertion elements, IS701 and IS702, isolated from Calothrix species PCC 7601. These insertion elements were cloned from spontaneous pigmentation mutants. Both show the characteristics of typical bacterial insertion sequences, i.e. they present long terminal inverted repeats and they duplicate target DNA upon insertion. These elements share no homology with the only other cyanobacterial insertion sequence described so far, IS891. At least 15 copies of IS701 and 9 copies of IS702 were detected by hybridization experiments in the Calothrix 7601 genome. Their occurrence in several cyanobacterial strains is also reported.

Control of nitrogen and carbon metabolism in cyanobacteria

In the unicellular cyanobacteria that do not fix molecular nitrogen, interactions between N assimilation and C metabolism occur through the signal transducer PII and the global nitrogen regulator NtcA. Under high CO2 concentration, PII liganded to ATP and boundto 2-oxoglutarate becomes phosphorylated and negatively controls the high affinity transport for bicarbonate. In contrast, under low CO2, PII being only liganded to ATP becomes dephosphorylated and negatively controls the nitrate/nitrite active transport system. The redox state of the cells together with NtcA also modulate the phosphorylation state of PII. Moreover, the regulation of the expression of the gene encoding PII is at least in part NthA-dependent. This network of transcriptional and post-transcriptional regulations allows cells to rapidly acclimate by adjusting their carbon and nitrogen metabolism in response to changes in environmental coditions.

Gas vesicle synthesis in the cyanobacterium Pseudanabaena sp.: occurrence of a single photoregulated gene

Gas vesicles are subcellular inclusions found in a large number of aquatic prokaryotes. The gvpA gene, which frequently occurs as a multigene family, encodes the major gas vesicle structural protein. In several cyanobacteria, another gene, gvpC, encodes a different protein which might be a dispensable element for gas vesicle formation. We report here the molecular characterization of a gvpA gene in Pseudanabaena sp. PCC 6901. In this planktonic cyanobacterium, it is the only gvp gene which could be detected, and electrophoretic analysis of isolated gas vesicles revealed the presence of a single protein. A monocistronic mRNA species corresponds to the transcription of the gvpA gene and the abundance of the gvpA mRNA is inversely correlated with photosynthetic photon flux density, indicating that a light‐dependent transcriptional regulation is likely to be involved in the control of gas vacuolation in this strain.

The Gas Vesicle Gene (gvp) Cluster of the Cyanobacterium Pseudanabaena sp. Strain PCC 6901

A gene cluster located downstream from gvpA in the cyanobacterium Pseudanabaena sp. strain PCC 6901 has been cloned and sequenced. The three genes, orfl, gvpN and gvpj, are consecutive with no intergenic region. In contrast to GvpN and Gvpj, which share high similarity at the amino acid level with their counterparts in other cyanobacteria and halophilic archaea, Orfl is only 29% identical to the C-terminal part of GvpC from Anabaena flos-aquae and its sequence organization is reminiscent of the halophilic archaeal GvpC.