N Tandeau de Marsac

A plasmid‐encoded gas vesicle protein gene in a halophilic archaebacterium

The halophilic archaebacterium, Halobacterium halobium, displays spontaneous and revertible genetic variability for the gas vesicle phenotype (Vac) at frequencies as high as 0.5 to 5%. To investigate the mechanism of these high‐frequency mutations, we have cloned a gas vesicle protein gene (gvpA) from the Vac+ wild‐type H. halobium strain, NRC‐1, and determined its nucleotide sequence, transcription start site, and genomic location. The gene sequence predicts that the gas vesicle protein has a molecular weight of 9156 and is relatively hydrophobic except for a hydrophilic C‐terminal region. Northern hybridization analysis shows that the gene is transcribed into a 350‐nucleotide mRNA, and primer extension analysis indicates that transcription begins 20 nucleotides upstream of the ATG start codon. Southern hybridization analysis shows that the gene is encoded by a large H. halobium plasmid. We discuss potential mechanisms for genetic variability of the Vac phenotype and identify sequences in the gvpA promoter region which may function as signals for transcription in H. halobium.

Stepwise effects of cytokinin activity and DNA synthesis upon mitotic cycle events in partially synchronized tobacco cells

Abstract Cytokinin addition to tobacco cell suspensions induced synchronous cell division after an 18 h lag period. Although continuous presence of the cytokinin in the culture medium during this lag period was essential to division, cytokinin was not required during mitosis itself. For each cell generation, cytokinin-dependent events are thus completed before mitosis occurs. Two experiments suggested that these cytokinin-dependent events are independent of DNA synthesis: 1. (i) With or without cytokinin, DNA synthesis proceeded normally in the presence of auxin, for at least the time required for one cell generation in complete medium. 2. (ii) In the presence of cytokinin, when DNA synthesis in the lag period was inhibited by FUdR, one normal cell division occurred when cytokinin was withdrawn and DNA synthesis restored by thymidine addition. In cytokinin-starved cells, metaphase was greatly prolonged although prophase was unaffected.

Organization and nucleotide sequence of genes encoding core components of the phycobilisomes from Synechococcus 6301.

SummaryCyanobacteria possess specialized organelles, called phycobilisomes, which collect and transfer light energy to the reaction centres of photosystem II, in the photosynthetic membrane. Phycobilisomes consist of a central core, mainly composed of allophycocyanin, from which six rods radiate. We report here the isolation, for the first time, of three genes that encode core components of cyanobacterial phycobilisomes. The genes coding for the α-and β-subunit apoproteins of allophycocyanin (apcA and apcB) were cloned from Synechococcus PCC 6301 and subjected to nucleotide sequence analysis. Dowstream of apcB, we found a third open reading frame (apcC) which, by comparison with known amino acid sequences, was assigned to Lc7.8, a linker polypeptide associated with phycobiliproteins within the core of the phycobilisomes. Homologies between amino acid sequences deduced from the nucleotide sequence of the Synechococcus PCC 6301 apc genes and the amino acid sequences published for corresponding proteins either from cyanobacteria or chloroplast-like organelles of eukaryotic organisms, are 75% or more. The genetic organization of this photosynthetic gene cluster relative to that observed in the cyanelle genome of the flagellate Cyanophora paradoxa is discussed.

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.

Genome Sequence of the Edible Cyanobacterium Arthrospira sp. PCC 8005

We determined the genome sequence of Arthrospira sp. PCC 8005, a cyanobacterial strain of great interest to the European Space Agency for its nutritive value and oxygenic properties in the Micro-Ecological Life Support System Alternative (MELiSSA) biological life support system for long-term manned missions into space.

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.

Isolation and molecular characterization of the gene encoding allophycocyanin B, a terminal energy acceptor in cyanobacterial phycobillsomes

Phycobilisomes are the major constituents of the light‐harvesting apparatus in both cyanobacteria and red algae and consist of a central core with radiating rods. From a genomic library of the cyanobacterium Calothrix 7601, a DNA fragment encoding allophycocyanin B, one of the two terminal energy acceptors of the core, was isolated and its nucleotide sequence was determined. Unlike all the other known genes encoding phycobiliproteins, the allophycocyanin B gene, apcD, is transcribed as a monocistronic unit. Mapping of the transcripts was performed and, in contrast to some of the Calothrix genes that encode rod components, transcription was shown to occur regardless of chromatic tight received during cell growth.

Réhabilitation par les microorganismes de sites contenant du pyralène: problématique et perspectives d'étude

Summary Polychlorinated biphenyls (PCB) have been widely used in a number of industrial applications for nearly 50 years. As a consequence of their intensive use, and due to the stability of the molecules, they have become progressively widespread contaminants in soil, sediments and landfills. Because classical remediation technologies (confinement, incineration, etc.) are generally expensive and not always efficient, alternative methods involving biodegradation by microbial populations have been developed. The two known pathways of biodegradation (oxidative and reductive), as well as factors affecting PCB degradation by microbial strains, are reviewed here. The different bioremediation strategies proposed and those recently developed are examined and compared. The use and study of new microorganisms, including cyanobacteria, are also evoked.

Isolation of a deoxycytidylate methyl transferase capable of protecting DNA uniquely against cleavage by endonuclease R.Aqu I (isoschizomer of Ava I)

A sequence-specific modification methylase (M.AquI) was isolated and purified from Agmenellum quadruplicatum (Synechococcus PCC 7002). This enzyme uniquely methylates the deoxycytidylate residue in the sequence *CYCGRG indicated by the asterisk. It was shown to protect DNA against cleavage by restriction endonucleases AvaI, SmaI and XhoI, which recognize the sequences CYCGRG, CCCGGG, and CTCGAG, respectively.