A day in the life of microcystis aeruginosa strain PCC 7806 as revealed by a transcriptomic analysis.

The cyanobacterium, Microcystis aeruginosa, is able to proliferate in a wide range of freshwater ecosystems and to produce many secondary metabolites that are a threat to human and animal health. The dynamic of this production and more globally the metabolism of this species is still poorly known. A DNA microarray based on the genome of M. aeruginosa PCC 7806 was constructed and used to study the dynamics of gene expression in this cyanobacterium during the light/dark cycle, because light is a critical factor for this species, like for other photosynthetic microorganisms. This first application of transcriptomics to a Microcystis species has revealed that more than 25% of the genes displayed significant changes in their transcript abundance during the light/dark cycle and in particular during the dark/light transition. The metabolism of M. aeruginosa is compartmentalized between the light period, during which carbon uptake, photosynthesis and the reductive pentose phosphate pathway lead to the synthesis of glycogen, and the dark period, during which glycogen degradation, the oxidative pentose phosphate pathway, the TCA branched pathway and ammonium uptake promote amino acid biosynthesis. We also show that the biosynthesis of secondary metabolites, such as microcystins, aeruginosin and cyanopeptolin, occur essentially during the light period, suggesting that these metabolites may interact with the diurnal part of the central metabolism.

Highly plastic genome of Microcystis aeruginosa PCC 7806, a ubiquitous toxic freshwater cyanobacterium.

BACKGROUND: The colonial cyanobacterium Microcystis proliferates in a wide range of freshwater ecosystems and is exposed to changing environmental factors during its life cycle. Microcystis blooms are often toxic, potentially fatal to animals and humans, and may cause environmental problems. There has been little investigation of the genomics of these cyanobacteria. RESULTS: Deciphering the 5,172,804 bp sequence of Microcystis aeruginosa PCC 7806 has revealed the high plasticity of its genome: 11.7% DNA repeats containing more than 1,000 bases, 6.8% putative transposases and 21 putative restriction enzymes. Compared to the genomes of other cyanobacterial lineages, strain PCC 7806 contains a large number of atypical genes that may have been acquired by lateral transfers. Metabolic pathways, such as fermentation and a methionine salvage pathway, have been identified, as have genes for programmed cell death that may be related to the rapid disappearance of Microcystis blooms in nature. Analysis of the PCC 7806 genome also reveals striking novel biosynthetic features that might help to elucidate the ecological impact of secondary metabolites and lead to the discovery of novel metabolites for new biotechnological applications. M. aeruginosa and other large cyanobacterial genomes exhibit a rapid loss of synteny in contrast to other microbial genomes. CONCLUSION: Microcystis aeruginosa PCC 7806 appears to have adopted an evolutionary strategy relying on unusual genome plasticity to adapt to eutrophic freshwater ecosystems, a property shared by another strain of M. aeruginosa (NIES-843). Comparisons of the genomes of PCC 7806 and other cyanobacterial strains indicate that a similar strategy may have also been used by the marine strain Crocosphaera watsonii WH8501 to adapt to other ecological niches, such as oligotrophic open oceans.

Microcyclamide biosynthesis in two strains of Microcystis aeruginosa: from structure to genes and vice versa.

Comparative analysis of related biosynthetic gene clusters can provide new insights into the versatility of these pathways and allow the discovery of new natural products. The freshwater cyanobacterium Microcystis aeruginosa NIES298 produces the cytotoxic peptide microcyclamide. Here, we provide evidence that the cyclic hexapeptide is formed by a ribosomal pathway through the activity of a set of processing enzymes closely resembling those recently shown to be involved in patellamide biosynthesis in cyanobacterial symbionts of ascidians. Besides two subtilisin-type proteases and a heterocyclization enzyme, the gene cluster discovered in strain NIES298 encodes six further open reading frames, two of them without similarity to enzymes encoded by the patellamide gene cluster. Analyses of genomic data of a second cyanobacterial strain, M. aeruginosa PCC 7806, guided the discovery and structural elucidation of two novel peptides of the microcyclamide family. The identification of the microcyclamide biosynthetic genes provided an avenue by which to study the regulation of peptide synthesis at the transcriptional level. The precursor genes were strongly and constitutively expressed throughout the growth phase, excluding the autoinduction of these peptides, as has been observed for several peptide pheromone families in bacteria.

Organ-targeted mutagenicity of nitrofurantoin in Big Blue transgenic mice.

Nitrofurans are widely used in human medicine, as nitrofurantoin and nifuroxazide, still prescribed for long-term antimicrobial prophylaxis of urinary tract and gastrointestinal infection in humans respectively. Recent experiments in mammals, as well as reports mentioning toxic effects in humans associated with a long-term use, specially in the case of nitrofurantoin, raised the need for reevaluating their genotoxicity. The objective of this study was to determine whether these two compounds induce a mutagenic effect in the Big Blue transgenic mouse mutation assay. Mice were orally treated either with nitrofurantoin or nifuroxazide for five consecutive days and sacrificed 3 weeks later. In order to optimize the genotoxic response, the doses used for each compound were 25-fold higher as the posology in humans. They corresponded to 50% of the highest doses tolerated by mice. The mutant frequency was determined from kidney, lung, bladder, caecum, colon, small intestine, spleen and stomach. A weak mutagenic response of nitrofurantoin-treated mice specifically in the kidney was observed. As in the case of other nitrofuran compounds, the mutation spectra determined from treated samples exhibited slightly more GC-->TA transversions as compared with untreated conditions. These data are relevant to the targeted action of nitrofurantoin as a urinary antimicrobial agent. No significant increase of mutants was detected in the case of nifuroxazide-treated mice whatever the organs analysed.

A new rubisco-like protein coexists with a photosynthetic rubisco in the planktonic cyanobacteria Microcystis.

Two genes encoding proteins related to large subunits of Rubisco were identified in the genome of the planktonic cyanobacterium Microcystis aeruginosa PCC 7806 that forms water blooms worldwide. The rbcL(I) gene belongs to the form I subfamily typically encountered in cyanobacteria, green algae, and land plants. The second and newly discovered gene is of the form IV subfamily and widespread in the Microcystis genus. In M. aeruginosa PCC 7806 cells, the expression of both rbcL(I) and rbcL(IV) is sulfur-dependent. The purified recombinant RbcL(IV) overexpressed in Escherichia coli cells did not display CO(2) fixation activity but catalyzed enolization of 2,3-diketo-5-methylthiopentyl-1-phosphate, and the rbcL(IV) gene rescued a Bacillus subtilis MtnW-deficient mutant. Therefore, the Microcystis RbcL(IV) protein functions both in vitro and in vivo and might be involved in a methionine salvage pathway. Despite variations in the amino acid sequences, RbcL(IV) shares structural similarities with all members of the Rubisco superfamily. Invariant amino acids within the catalytic site may thus represent the minimal set for enolization, whereas variations, especially located in loop 6, may account for the limitation of the catalytic reaction to enolization. Even at low protein concentrations in vitro, the recombinant RbcL(IV) assembles spontaneously into dimers, the minimal unit required for Rubisco forms I-III activity. The discovery of the coexistence of RbcL(I) and RbcL(IV) in cyanobacteria, the ancestors of chloroplasts, enlightens episodes of the chaotic evolutionary history of the Rubiscos, a protein family of major importance for life on Earth.

DNA array analysis of gene expression in response to UV irradiation in Escherichia coli.

The capacity of DNA macroarrays that contain all 4290 predicted open reading frames of the E. coli K12 genome was evaluated by measuring changes in gene expression in response to irradiation by ultraviolet light (UV). UV and other DNA damaging agents are known to trigger the induction of the SOS response. This is a coordinated increase in the level of expression of a set of approximately 30 unlinked genes, the SOS genes, negatively regulated by the LexA repressor. The analysis was performed on a set of isogenic strains with mutations that affect expression of genes of the SOS system: (i) the lexA+ strain, in which the SOS system can be induced after DNA damage, (ii) lexAind- mutants in which the SOS system cannot be induced, and (iii) lexAdef mutants in which the SOS system is induced constitutively. We found that a large set of genes appeared to be either upregulated or downregulated following UV irradiation. Among the genes which appeared to be upregulated in a LexA-dependent manner, we correctly identified 9 out of 27 SOS genes printed on the arrays and one gene containing a LexA binding site. One gene, dnaN, encoding the beta subunit of DNA polymerase III holoenzyme, was identified as an upregulated gene in a LexA-independent manner. Our results were compared to those of similar studies previously published. Although the SOS response as a whole could not be illustrated by using DNA arrays, the data suggest that regulation of some SOS genes might be more complex than previously thought.

Genotoxicity of 2-nitro-7-methoxy-naphtho[2,1-b]furan (R7000): a case study with some considerations on nitrofurantoin and nifuroxazide.

Two nitrofurans present broad-spectrum antimicrobial properties and some of them are used in human and veterinary medicine. Most of these molecules are mutagens and some of them were reported as carcinogens. Due to its extreme mutagenic potency in bacteria, the nitronaphtho derivative 2-nitro-7-methoxy-naphtho[2,1-b]furan (R7000) was used as a tool to analyze the mechanism of the genotoxic action of this family of chemicals. In the present paper, we review essential data on the genotoxicity of R7000 and briefly discuss the case of nitrofurantoin and nifuroxazide, two nitrofurans, still in use as urinary and gastrointestinal disinfectants.

Comparison of kinetics of induction of DNA adducts and gene mutations by a nitrofuran compound, 7-methoxy-2-nitronaphtho[2,1-b]furan (R7000), in the caecum and small intestine of Big Blue mice.

In previous experiments, i.p. injection of the 5 nitronaphthofuran derivative 7-methoxy-2-nitronaphtho[2,1-b]furan (R7000) to lacI transgenic Big Blue mice led to an increase in the mutant frequency (MF), especially in the caecum and the small intestine. In the present work, the in vivo genotoxicity of R7000 in these two target organs was further investigated. Big Blue mice were treated with a single daily i.p. injection of R7000 of 0.05-0.5 mg/day for five consecutive days and killed 28 days later. These treatments led to significant increases in MF of 1.8-, 3- and 5.4-fold at 0.1, 0.2 and 0.5 mg/day R7000, respectively, in the small intestine. In the caecum, a mutagenic effect, of 4.5-fold, was only observed at the highest dose. DNA adduct formation and MFs resulting from R7000 were also analysed in parallel at various times after the last injection. R7000 led to 14 and seven different nucleotide modifications in the caecum and small intestine, respectively. Three hours after the final injection the level of induced DNA adducts was 10 times higher in the caecum than in the small intestine. From 3 h to 5 days after the final injection, 93 and 58% of DNA adducts disappeared in the caecum and small intestine, respectively. The resulting MF values were similar when comparing the two organs. Analysis of the R7000-induced mutation spectrum in the caecum showed that single G:C and large, > or =3 bp deletions and GC-->CG transversions were the first induced mutations at the end of the treatment. Fifteen days later, the R7000 mutation specificity characteristics already reported in Escherichia coli and in the small intestine of Big Blue mice were evident in the caecum, with the two major events being GC-->TA transversions and deletions of one G:C base pair. In both organs, a relationship between the decrease in R7000-DNA adducts and induction of MF was evident. However, the efficiency of this compound in damaging DNA was not correlated with the capacity of DNA lesions to lead to mutations. Some discrepancies in the R7000 genotoxic effects between the two organs were observed, which may be attributable to differences in the metabolic activation pathway of the compound, as well as to DNA repair proficiency in each tissue.