Chlamydomonas reinhardtii and Microbacterium forte sp. nov., a mutualistic association that favors sustainable hydrogen production

A naturally occurring multispecies bacterial community composed of Bacillus cereus and two novel bacteria (Microbacterium forte sp. nov. and Stenotrophomonas goyi sp. nov.) has been identified from a contaminated culture of the microalga Chlamydomonas reinhardtii. When incubated in mannitol- and yeast extract-containing medium, this bacterial community can promote and sustain algal hydrogen production up to 313 mL H2·L-1 for 17 days and 163.5 mL H2·L-1 for 25 days in high-cell (76.7 μg·mL-1 of initial chlorophyll) and low-cell density (10 μg·mL-1 of initial chlorophyll) algal cultures, respectively. In low-cell density algal cultures, hydrogen production was compatible with algal growth (reaching up to 60 μg·mL-1 of chlorophyll). Among the bacterial community, M. forte sp. nov. was the sole responsible for the improvement in hydrogen production. However, algal growth was not observed in the Chlamydomonas-M. forte sp. nov. consortium during hydrogen-producing conditions (hypoxia), suggesting that the presence of B. cereus and S. goyi sp. nov. could be crucial to support the algal growth during hypoxia. Still, under non‑hydrogen producing conditions (aerobiosis) the Chlamydomonas-M. forte sp. nov. consortium allowed algal growth (up to 40 μg·mL-1 of chlorophyll) and long-term algal viability (>45 days). The genome sequence and growth tests of M. forte sp. nov. have revealed that this bacterium is auxotroph for biotin and thiamine and unable to use sulfate as sulfur source; it requires S-reduced forms such as cysteine and methionine. Cocultures of Chlamydomonas reinhardtii and M. forte sp. nov. established a mutualistic association: the alga complemented the nutrient deficiencies of the bacterium, while the bacterium released ammonium (0.19 mM·day-1) and acetic acid (0.15 mM·day-1) for the alga. This work offers a promising avenue for photohydrogen production concomitant with algal biomass generation using nutrients not suitable for mixotrophic algal growth.

Stenotrophomonas goyi sp. nov., a novel bacterium associated with the alga Chlamydomonas reinhardtii

Background

A culture of the green algae Chlamydomonas reinhardtii was accidentally contaminated with three different bacteria in our laboratory facilities. This contaminated alga culture showed increased algal biohydrogen production. These three bacteria were independently isolated.

Methods

The chromosomic DNA of one of the isolated bacteria was extracted and sequenced using PacBio technology. Tentative genome annotation (RAST server) and phylogenetic trees analysis (TYGS server) were conducted. Diverse growth tests were assayed for the bacterium and for the alga-bacterium consortium.

Results

Phylogenetic analysis indicates that the bacterium is a novel member of the Stenotrophomonas genus that has been termed in this work as S. goyi sp. nov. A fully sequenced genome (4,487,389 base pairs) and its tentative annotation (4,147 genes) are provided. The genome information suggests that S. goyi sp. nov. is unable to use sulfate and nitrate as sulfur and nitrogen sources, respectively. Growth tests have confirmed the dependence on the sulfur-containing amino acids methionine and cysteine. S. goyi sp. nov. and Chlamydomonas reinhardtii can establish a mutualistic relationship when cocultured together.

Conclusions

S. goyi sp. nov. could be of interest for the design of biotechnological approaches based on the use of artificial microalgae-bacteria multispecies consortia that take advantage of the complementary metabolic capacities of their different microorganisms.

Stenotrophomonas goyi sp. nov., a novel bacterium associated with the alga Chlamydomonas reinhardtii

BACKGROUND

A culture of the green algae Chlamydomonas reinhardtii was accidentally contaminated with three different bacteria in our laboratory facilities. This contaminated alga culture showed increased algal biohydrogen production. These three bacteria were independently isolated.

METHODS

The chromosomic DNA of one of the isolated bacteria was extracted and sequenced using PacBio technology. Tentative genome annotation (RAST server) and phylogenetic trees analysis (TYGS server) were conducted. Diverse growth tests were assayed for the bacterium and for the alga-bacterium consortium.

RESULTS

Phylogenetic analysis indicates that the bacterium is a novel member of the Stenotrophomonas genus that has been termed in this work as S. goyi sp. nov. A fully sequenced genome (4,487,389 base pairs) and its tentative annotation (4,147 genes) are provided. The genome information suggests that S. goyi sp. nov. is unable to use sulfate and nitrate as sulfur and nitrogen sources, respectively. Growth tests have confirmed the dependence on the sulfur-containing amino acids methionine and cysteine. S. goyi sp. nov. and Chlamydomonas reinhardtii can establish a mutualistic relationship when cocultured together.

CONCLUSIONS

S. goyi sp. nov. could be of interest for the design of biotechnological approaches based on the use of artificial microalgae-bacteria multispecies consortia that take advantage of the complementary metabolic capacities of their different microorganisms.

Chlamydomonas-Methylobacterium oryzae cooperation leads to increased biomass, nitrogen removal and hydrogen production

In the context of algal wastewater bioremediation, this study has identified a novel consortium formed by the bacterium Methylobacterium oryzae and the microalga Chlamydomonas reinhardtii that greatly increase biomass generation (1.22 g L^-1·d^-1), inorganic nitrogen removal (>99%), and hydrogen production (33 mL·L^-1) when incubated in media containing ethanol and methanol. The key metabolic aspect of this relationship relied on the bacterial oxidation of ethanol to acetate, which supported heterotrophic algal growth. However, in the bacterial monocultures the acetate accumulation inhibited bacterial growth. Moreover, in the absence of methanol, ethanol was an unsuitable carbon source and its incomplete oxidation to acetaldehyde had a toxic effect on both the alga and the bacterium. In cocultures, both alcohols were used as carbon sources by the bacteria, the inhibitory effects were overcome and both microorganisms mutually benefited. Potential biotechnological applications in wastewater treatment, biomass generation and hydrogen production are discussed.

Acetic acid is key for synergetic hydrogen production in Chlamydomonas-bacteria co-cultures

This study is a proof of concept for the synergetic biohydrogen production in alga-bacteria co-cultures. Algal hydrogen photoproduction was obtained in sugar-containing media only when the green alga Chlamydomonas reinhardtii was co-cultured with Pseudomonas putida (40.8 ml H₂·L^-1), Escherichia coli (35.1 ml H₂·L^-1) and Rhizobium etli (16.1 ml H₂·L^-1). Hydrogen photo-production in these co-cultures was not only linked to the induction of hypoxia, but to the ability of the bacteria to produce acetic acid from sugars. Synergetic hydrogen production was achieved by integrating the photobiological and fermentative production in Chlamydomonas and Escherichia coli co-cultures supplemented with glucose, which resulted in 60% more H₂ production than the sum of the respective monocultures. This cooperation relied on the ability of the alga to consume the excreted bacterial acetic acid, which benefited both bacterial and algal hydrogen production. This knowledge may open new possibilities for the biohydrogen production from industrial wastes.

Ferredoxin5 Deletion Affects Metabolism of Algae during the Different Phases of Sulfur Deprivation

Ferredoxin5 (FDX5), a minor ferredoxin protein in the alga Chlamydomonas (Chlamydomonas reinhardtii), helps maintain thylakoid membrane integrity in the dark. Sulfur (S) deprivation has been used to achieve prolonged hydrogen production in green algae. Here, we propose that FDX5 is involved in algal responses to S-deprivation as well as to the dark. Specifically, we tested the role of FDX5 in both the initial aerobic and subsequent anaerobic phases of S-deprivation. Under S-deprived conditions, absence of FDX5 causes a distinct delay in achieving anoxia by affecting photosynthetic O2 evolution, accompanied by reduced acetate uptake, lower starch accumulation, and delayed/lower fermentative metabolite production, including photohydrogen. We attribute these differences to transcriptional and/or posttranslational regulation of acetyl-CoA synthetase and ADP-Glc pyrophosphorylase, and increased stability of the PSII D1 protein. Interestingly, increased levels of FDX2 and FDX1 were observed in the mutant under oxic, S-replete conditions, strengthening our previously proposed hypothesis that other ferredoxins compensate in response to a lack of FDX5. Taken together, the results of our omics and pull-down experiments confirmed biochemical and physiological results, suggesting that FDX5 may have other effects on Chlamydomonas metabolism through its interaction with multiple redox partners.

Arginine is a component of the ammonium-CYG56 signalling cascade that represses genes of the nitrogen assimilation pathway in Chlamydomonas reinhardtii

Nitrogen assimilation and metabolism are essential processes for all living organisms, yet there is still much to be learnt on how they are regulated. The use of Chlamydomonas reinhardtii as a model system has been instrumental not only in identifying conserved regulation mechanisms that control the nitrogen assimilation pathway, but also in understanding how the intracellular nitrogen status regulates metabolic processes of industrial interest such as the synthesis of biolipids. While the genetic regulators that control the nitrogen pathway are successfully being unravelled, other layers of regulation have received less attention. Amino acids, for example, regulate nitrogen assimilation in certain organisms, but their role in Chlamydomonas has not thoroughly been explored. Previous results had suggested that arginine might repress key genes of the nitrogen assimilation pathway by acting within the ammonium negative signalling cascade, upstream of the nitric oxide (NO) inducible guanylate cyclase CYG56. We tested this hypothesis with a combination of genetic and chemical approaches. Antagonising the effects of arginine with an arginine biosynthesis mutant or with two chemical analogues released gene expression from ammonium mediated repression. The cyg56 and related non1 mutants, which are partially insensitive to ammonium repression, were also partially insensitive to repression by arginine. Finally, we show that the addition of arginine to the medium leads to an increase in intracellular NO. Our data reveal that arginine acts as a negative signal for the assimilation of nitrogen within the ammonium-CYG56 negative signalling cascade, and provide a connection between amino acid metabolism and nitrogen assimilation in microalgae.

H2 production pathways in nutrient-replete mixotrophic Chlamydomonas cultures under low light. Response to the commentary article "On the pathways feeding the H2 production process in nutrient-replete, hypoxic conditions," by Alberto Scoma and Szilvia Z. Tóth

BACKGROUND

A recent Commentary article entitled "On the pathways feeding the H₂ production process in nutrient-replete, hypoxic conditions" by Dr. Scoma and Dr. Tóth, Biotechnology for Biofuels (2017), opened a very interesting debate about the H₂ production photosynthetic-linked pathways occurring in Chlamydomonas cultures grown in acetate-containing media and incubated under hypoxia/anoxia conditions. This Commentary article mainly focused on the results of our previous article "Low oxygen levels contribute to improve photohydrogen production in mixotrophic non-stressed Chlamydomonas cultures," by Jurado-Oller et al., Biotechnology for Biofuels (7, 2015; 8:149).

MAIN BODY

Here, we review some previous knowledge about the H₂ production pathways linked to photosynthesis in Chlamydomonas, especially focusing on the role of the PSII-dependent and -independent pathways in acetate-containing nutrient-replete cultures. The potential contributions of these pathways to H₂ production under anoxia/hypoxia are discussed.

CONCLUSION

Despite the fact that the PSII inhibitor DCMU is broadly used to discern between the two different photosynthetic pathways operating under H₂ production conditions, its use may lead to distinctive conclusions depending on the growth conditions. The different potential sources of reductive power needed for the PSII-independent H₂ production in mixotrophic nutrient-replete cultures are a matter of debate and conclusive evidences are still missing.

Glucose Uptake in Prochlorococcus: Diversity of Kinetics and Effects on the Metabolism

We have previously shown that Prochlorococcus sp. SS120 strain takes up glucose by using a multiphasic transporter encoded by the Pro1404 gene. Here, we studied the glucose uptake kinetics in multiple Prochlorococcus strains from different ecotypes, observing diverse values for the K_s constants (15–126.60 nM) and the uptake rates (0.48–6.36 pmol min^-1 mg prot^-1). Multiphasic kinetics was observed in all studied strains, except for TAK9803-2. Pro1404 gene expression studies during the 21st Atlantic Meridional Transect cruise showed positive correlation with glucose concentrations in the ocean. This suggests that the Pro1404 transporter has been subjected to diversification along the Prochlorococcus evolution, in a process probably driven by the glucose availabilities at the different niches it inhabits. The glucose uptake mechanism seems to be a primary transporter. Glucose addition induced detectable transcriptomic and proteomic changes in Prochlorococcus SS120, but photosynthetic efficiency was unaffected. Our studies indicate that glucose is actively taken up by Prochlorococcus, but its uptake does not significantly alter the trophic ways of this cyanobacterium, which continues performing photosynthesis. Therefore Prochlorococcus seems to remain acting as a fundamentally phototrophic organism, capable of using glucose as an extra resource of carbon and energy when available in the environment.

Functional reconstruction after subtotal glossectomy in the surgical treatment of an uncommon and aggressive neoplasm in this location: Primary malignant melanoma in the base of the tongue

Primary malignant melanoma of the oral cavity is a rare neoplasm, especially on the tongue. We report a case of mucosal melanoma at the base of the tongue, an extremely rare location (only about 30 cases have been reported in literature). The extension study doesn´t revealed distant metastatic lesions. The patient was treated by subtotal glossectomy and bilateral functional neck dissection. Tongue is one of the most difficult structures to reconstruct, because of their central role in phonation, swallowing and airway protection. The defect was reconstructed with anterolateral thigh free flap. Surgical treatment was supplemented with adjuvant immunotherapy. The post-operative period was uneventful. At present, 24 months after surgery, patient is asymptomatic, there isn´t evidence of recurrence of melanoma and he hasn´t any difficulty in swallowing or phonation.

Key words:Malignant mucosal melanoma, anterolateral thigh free flap, phonation, swallowing.

Low oxygen levels contribute to improve photohydrogen production in mixotrophic non-stressed Chlamydomonas cultures

BACKGROUND

Currently, hydrogen fuel is derived mainly from fossil fuels, but there is an increasing interest in clean and sustainable technologies for hydrogen production. In this context, the ability of some photosynthetic microorganisms, particularly cyanobacteria and microalgae, to produce hydrogen is a promising alternative for renewable, clean-energy production. Among a diverse array of photosynthetic microorganisms able to produce hydrogen, the green algae Chlamydomonas reinhardtii is the model organism widely used to study hydrogen production. Despite the well-known fact that acetate-containing medium enhances hydrogen production in this algae, little is known about the precise role of acetate during this process.

RESULTS

We have examined several physiological aspects related to acetate assimilation in the context of hydrogen production metabolism. Measurements of oxygen and CO2 levels, acetate uptake, and cell growth were performed under different light conditions, and oxygenic regimes. We show that oxygen and light intensity levels control acetate assimilation and modulate hydrogen production. We also demonstrate that the determination of the contribution of the PSII-dependent hydrogen production pathway in mixotrophic cultures, using the photosynthetic inhibitor DCMU, can lead to dissimilar results when used under various oxygenic regimes. The level of inhibition of DCMU in hydrogen production under low light seems to be linked to the acetate uptake rates. Moreover, we highlight the importance of releasing the hydrogen partial pressure to avoid an inherent inhibitory factor on the hydrogen production.

CONCLUSION

Low levels of oxygen allow for low acetate uptake rates, and paradoxically, lead to efficient and sustained production of hydrogen. Our data suggest that acetate plays an important role in the hydrogen production process, during non-stressed conditions, other than establishing anaerobiosis, and independent of starch accumulation. Potential metabolic pathways involved in hydrogen production in mixotrophic cultures are discussed. Mixotrophic nutrient-replete cultures under low light are shown to be an alternative for the simultaneous production of hydrogen and biomass.

Tiered regulation of sulfur deprivation responses in Chlamydomonas reinhardtii and identification of an associated regulatory factor

During sulfur (S) deprivation, the unicellular alga Chlamydomonas reinhardtii exhibits increased expression of numerous genes. These genes encode proteins associated with sulfate (SO4(2-)) acquisition and assimilation, alterations in cellular metabolism, and internal S recycling. Administration of the cytoplasmic translational inhibitor cycloheximide prevents S deprivation-triggered accumulation of transcripts encoding arylsulfatases (ARS), an extracellular polypeptide that may be important for cell wall biosynthesis (ECP76), a light-harvesting protein (LHCBM9), the selenium-binding protein, and the haloperoxidase (HAP2). In contrast, the rapid accumulation of transcripts encoding high-affinity SO4(2-) transporters is not affected. These results suggest that there are two tiers of transcriptional regulation associated with S deprivation responses: the first is protein synthesis independent, while the second requires de novo protein synthesis. A mutant designated ars73a exhibited low ARS activity and failed to show increases in ECP76, LHCBM9, and HAP2 transcripts (among others) in response to S deprivation; increases in transcripts encoding the SO4(2-) transporters were not affected. These results suggest that the ARS73a protein, which has no known activity but might be a transcriptional regulator, is required for the expression of genes associated with the second tier of transcriptional regulation. Analysis of the ars73a strain has helped us generate a model that incorporates a number of complexities associated with S deprivation responses in C. reinhardtii.

Giant cell granuloma of the maxilla. Global management, review of literature and case report

Giant cell granuloma is a relatively rare benign entity but can be locally aggressive. Histologically characterized by intense proliferation of multinucleated giant cells and fibroblasts. Affects bone supported tissues. Definitive diagnosis is given by biopsy. Clinically manifest as a mass or nodule of reddish color and fleshy, occasionally ulcerated surface. They can range from asymptomatic to destructive lesions that grow quickly. It is a lesion to be considered in the differential diagnosis of osteolytic lesions affecting the maxilla or jaw. Its management passed from conservative treatment with intralesional infiltration of corticosteroids, calcitonin or interferon, to the surgical resection and reconstruction, for example with microvascular free flaps.

Key words:Giant cell granuloma, intralesional injection, microvascular free flap, fibula.

RNA-seq analysis of sulfur-deprived Chlamydomonas cells reveals aspects of acclimation critical for cell survival

The Chlamydomonas reinhardtii transcriptome was characterized from nutrient-replete and sulfur-depleted wild-type and snrk2.1 mutant cells. This mutant is null for the regulatory Ser-Thr kinase SNRK2.1, which is required for acclimation of the alga to sulfur deprivation. The transcriptome analyses used microarray hybridization and RNA-seq technology. Quantitative RT-PCR evaluation of the results obtained by these techniques showed that RNA-seq reports a larger dynamic range of expression levels than do microarray hybridizations. Transcripts responsive to sulfur deprivation included those encoding proteins involved in sulfur acquisition and assimilation, synthesis of sulfur-containing metabolites, Cys degradation, and sulfur recycling. Furthermore, we noted potential modifications of cellular structures during sulfur deprivation, including the cell wall and complexes associated with the photosynthetic apparatus. Moreover, the data suggest that sulfur-deprived cells accumulate proteins with fewer sulfur-containing amino acids. Most of the sulfur deprivation responses are controlled by the SNRK2.1 protein kinase. The snrk2.1 mutant exhibits a set of unique responses during both sulfur-replete and sulfur-depleted conditions that are not observed in wild-type cells; the inability of this mutant to acclimate to S deprivation probably leads to elevated levels of singlet oxygen and severe oxidative stress, which ultimately causes cell death. The transcriptome results for wild-type and mutant cells strongly suggest the occurrence of massive changes in cellular physiology and metabolism as cells become depleted for sulfur and reveal aspects of acclimation that are likely critical for cell survival.

Insertional mutagenesis as a tool to study genes/functions in Chlamydomonas

The unicellular alga Chlamydomonas reinhardtii has emerged during the last decades as a model system to understand gene functions, many of them shared by bacteria, fungi, plants, animals and humans. A powerful resource for the research community is the availability of complete collections of stable mutants for studying whole genome function. In the meantime other strategies might be developed; insertional mutagenesis has become currently the best strategy to disrupt and tag nuclear genes in Chlamydomonas allowing forward and reverse genetic approaches. Here, we outline the mutagenesis technique stressing the idea of generating databases for ordered mutant libraries, and also of improving efficient methods for reverse genetics to identify mutants defective in a particular gene.

Nitrate signaling by the regulatory gene NIT2 in Chlamydomonas

Positive signaling by nitrate in its assimilation pathway has been studied in Chlamydomonas reinhardtii. Among >34,000 lines generated by plasmid insertion, 10 mutants were unable to activate nitrate reductase (NIA1) gene expression and had a Nit(-) (no growth in nitrate) phenotype. Each of these 10 lines was mutated in the nitrate assimilation-specific regulatory gene NIT2. The complete NIT2 cDNA sequence was obtained, and its deduced amino acid sequence revealed GAF, Gln-rich, Leu zipper, and RWP-RK domains typical of transcription factors and transcriptional coactivators associated with signaling pathways. The predicted Nit2 protein sequence is structurally related to the Nin (for nodule inception) proteins from plants but not to NirA/Nit4/Yna proteins from fungi and yeast. NIT2 expression is negatively regulated by ammonium and is optimal in N-free medium with no need for the presence of nitrate. However, intracellular nitrate is required to allow Nit2 to activate the NIA1 promoter activity. Nit2 protein was expressed in Escherichia coli and shown to bind to specific sequences at the NIA1 gene promoter. Our data indicate that NIT2 is a central regulatory gene required for nitrate signaling on the Chlamydomonas NIA1 gene promoter and that intracellular nitrate is needed for NIT2 function and to modulate NIA1 transcript levels.

The Chlamydomonas genome reveals the evolution of key animal and plant functions

Chlamydomonas reinhardtii is a unicellular green alga whose lineage diverged from land plants over 1 billion years ago. It is a model system for studying chloroplast-based photosynthesis, as well as the structure, assembly, and function of eukaryotic flagella (cilia), which were inherited from the common ancestor of plants and animals, but lost in land plants. We sequenced the approximately 120-megabase nuclear genome of Chlamydomonas and performed comparative phylogenomic analyses, identifying genes encoding uncharacterized proteins that are likely associated with the function and biogenesis of chloroplasts or eukaryotic flagella. Analyses of the Chlamydomonas genome advance our understanding of the ancestral eukaryotic cell, reveal previously unknown genes associated with photosynthetic and flagellar functions, and establish links between ciliopathy and the composition and function of flagella.

The Chlamydomonas genome reveals the evolution of key animal and plant functions

Chlamydomonas reinhardtii is a unicellular green alga whose lineage diverged from land plants over 1 billion years ago. It is a model system for studying chloroplast-based photosynthesis, as well as the structure, assembly, and function of eukaryotic flagella (cilia), which were inherited from the common ancestor of plants and animals, but lost in land plants. We sequenced the approximately 120-megabase nuclear genome of Chlamydomonas and performed comparative phylogenomic analyses, identifying genes encoding uncharacterized proteins that are likely associated with the function and biogenesis of chloroplasts or eukaryotic flagella. Analyses of the Chlamydomonas genome advance our understanding of the ancestral eukaryotic cell, reveal previously unknown genes associated with photosynthetic and flagellar functions, and establish links between ciliopathy and the composition and function of flagella.

Chlamydomonas reinhardtii CNX1E reconstitutes molybdenum cofactor biosynthesis in Escherichia coli mutants

We have isolated and characterized the Chlamydomonas reinhardtii genes for molybdenum cofactor biosynthesis, namely, CNX1G and CNX1E, and expressed them and their chimeric fusions in Chlamydomonas and Escherichia coli. In all cases, the wild-type phenotype was restored in individual mutants as well as in a CNX1G CNX1E double mutant. Therefore, CrCNX1E is the first eukaryotic protein able to complement an E. coli moeA mutant.

Restriction enzyme site-directed amplification PCR: A tool to identify regions flanking a marker DNA

An innovative combination of various recently described molecular methods was set up to efficiently identify regions flanking a marker DNA in insertional mutants of Chlamydomonas. The technique is named restriction enzyme site-directed amplification PCR (RESDA-PCR) and is based on the random distribution of frequent restriction sites in a genome and on a special design of primers. The primer design is based on the presence of a restriction site included in a low degenerated sequence at the 3' end and of a specific adapter sequence at the 5' end, with the two ends being linked by a polyinosine bridge. Specific primers of the marker DNA combined with the degenerated primers allow amplification of DNA fragments adjacent to the marker insertion by using two rounds of either short or long cycling procedures. Amplified fragments from 0.3 to 2 kb or more are routinely obtained at sufficient purity and quantity for direct sequencing. This method is fast, is reliable (87% success rate), and can be easily extrapolated to any organism and marker DNA by designing the appropriate primers. A procedure involving the PCR over enzyme digest fragments is also proposed for when, exceptionally, positive results are not obtained.

Functional genomics of the regulation of the nitrate assimilation pathway in Chlamydomonas

The existence of mutants at specific steps in a pathway is a valuable tool of functional genomics in an organism. Heterologous integration occurring during transformation with a selectable marker in Chlamydomonas (Chlamydomonas reinhardtii) has been used to generate an ordered mutant library. A strain, having a chimeric construct (pNia1::arylsulfatase gene) as a sensor of the Nia1 gene promoter activity, was transformed with a plasmid bearing the paramomycin resistance AphVIII gene to generate insertional mutants defective at regulatory steps of the nitrate assimilation pathway. Twenty-two thousand transformants were obtained and maintained in pools of 96 for further use. The mutant library was screened for the following phenotypes: insensitivity to the negative signal of ammonium, insensitivity to the positive signal of nitrate, overexpression in nitrate, and inability to use nitrate. Analyses of mutants showed that (1) the number or integrated copies of the gene marker is close to 1; (2) the probability of cloning the DNA region at the marker insertion site is high (76%); (3) insertions occur randomly; and (4) integrations at different positions and orientations of the same genomic region appeared in at least three cases. Some of the mutants analyzed were found to be affected at putative new genes related to regulatory functions, such as guanylate cyclase, protein kinase, peptidyl-prolyl isomerase, or DNA binding. The Chlamydomonas mutant library constructed would also be valuable to identify any other gene with a screenable phenotype.

Ammonium transporter genes in Chlamydomonas: the nitrate-specific regulatory gene Nit2 is involved in Amt1;1 expression

Ammonium transport is a key process in nitrogen metabolism. In the green alga Chlamydomonas, we have characterized molecularly the largest family of ammonium transporters (AMT1) so far described consisting of eight members. CrAmt1 genes have an interesting transcript structure with some very small exons. Differential expression patterns were found for each CrAmt1 gene in response to the nitrogen source by using Real Time PCR. These expression patterns were similar under high and low CO2 atmosphere. CrAmt1;1 expression was characterized in detail. It was repressed in both ammonium and nitrate medium, and strongly expressed in nitrogen-free media. Treatment with a Glutamine synthetase inhibitor released partially repression in ammonium and nitrate suggesting that ammonium and its derivatives participate in the observed repressing effects. By studying CrAmt1;1 expression in mutants deficient at different steps of the nitrate assimilation pathway, it has been shown that nitrate has a double negative effect on this gene expression; one related to its reduction to ammonium, and a second one by itself. This second effect of nitrate was dependent on the functionality of the regulatory gene Nit2, specific for nitrate assimilation. Thus, NIT2 would have a dual role on gene expression: the well-known positive one on nitrate assimilation and a novel negative one on Amt1;1 regulation.