DNA-mediated transformation of Chlamydomonas reinhardi cells: use of aminoglycoside 3'-phosphotransferase as a selectable marker

Identification of antibiotics for use in selection of the chytrid fungi Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans

Global amphibian populations are being decimated by chytridiomycosis, a deadly skin infection caused by the fungal pathogens Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal). Although ongoing efforts are attempting to limit the spread of these infections, targeted treatments are necessary to manage the disease. Currently, no tools for genetic manipulation are available to identify and test specific drug targets in these fungi. To facilitate the development of genetic tools in Bd and Bsal, we have tested five commonly used antibiotics with available resistance genes: Hygromycin, Blasticidin, Puromycin, Zeocin, and Neomycin. We have identified effective concentrations of each for selection in both liquid culture and on solid media. These concentrations are within the range of concentrations used for selecting genetically modified cells from a variety of other eukaryotic species.

Genome analysis and genetic transformation of a water surface-floating microalga Chlorococcum sp. FFG039

Microalgal harvesting and dewatering are the main bottlenecks that need to be overcome to tap the potential of microalgae for production of valuable compounds. Water surface-floating microalgae form robust biofilms, float on the water surface along with gas bubbles entrapped under the biofilms, and have great potential to overcome these bottlenecks. However, little is known about the molecular mechanisms involved in the water surface-floating phenotype. In the present study, we analysed the genome sequence of a water surface-floating microalga Chlorococcum sp. FFG039, with a next generation sequencing technique to elucidate the underlying mechanisms. Comparative genomics study with Chlorococcum sp. FFG039 and other non-floating green microalgae revealed some of the unique gene families belonging to this floating microalga, which may be involved in biofilm formation. Furthermore, genetic transformation of this microalga was achieved with an electroporation method. The genome information and transformation techniques presented in this study will be useful to obtain molecular insights into the water surface-floating phenotype of Chlorococcum sp. FFG039.

The bacterial phleomycin resistance gene ble as a dominant selectable marker in Chlamydomonas

A chimeric gene composed of the coding sequence of the ble gene from Streptoalloteichus hindustanus fused to the 5' and 3' untranslated regions of the Chlamydomonas reinhardtii nuclear gene RBCS2 has been constructed. Introduction of this chimeric gene into the nuclear genome of C. reinhardtii by co-transformation with the ARG7 marker yields Arg+ transformants of which approximately 80% possess the ble gene. Of these co-transformants, approximately 3% display a phleomycin-resistant (PmR) phenotype. Western blot analysis using antibodies against the ble gene product confirms the presence of the protein in the PmR transformants and genetic analysis demonstrates the co-segregation of the ble gene with the phenotype in progeny arising from the mating of a PmR transformant to wild-type strains. Direct selection of PmR transformants was achieved by allowing an 18-h period for recovery and growth of transformed cells prior to selection. This work represents the first demonstration of stable expression and inheritance of a foreign gene in the nuclear genome of C. reinhardtii and provides a useful dominant marker for nuclear transformation.

The CRY1 gene in Chlamydomonas reinhardtii: structure and use as a dominant selectable marker for nuclear transformation

We have cloned and sequenced the CRY1 gene, encoding ribosomal protein S14 in Chlamydomonas reinhardtii, and found that it is highly similar to S14/rp59 proteins from other organisms, including mammals, Drosophila melanogaster, and Saccharomyces cerevisiae. We isolated a mutant strain resistant to the eukaryotic translational inhibitors cryptopleurine and emetine in which the resistance was due to a missense mutation (CRY1-1) in the CRY1 gene; resistance was dominant in heterozygous stable diploids. Cotransformation experiments using the CRY1-1 gene and the gene for nitrate reductase (NIT1) produced a low level of resistance to cryptopleurine and emetine. Resistance levels were increased when the CRY1-1 gene was placed under the control of a constitutive promoter from the ribulose bisphosphate carboxylase/oxygenase small subunit 2 (RBCS2) gene. We also found that the 5' untranslated region of the CRY1 gene was required for expression of the CRY1-1 transgene. Direct selection of emetine-resistant transformants was possible when transformed cells were first induced to differentiate into gametes by nitrogen starvation and then allowed to dedifferentiate back to vegetative cells before emetine selection was applied. With this transformation protocol, the RBCS2/CRY1-1 dominant selectable marker gene is a powerful tool for many molecular genetic applications in C. reinhardtii.

The CRY1 gene in Chlamydomonas reinhardtii: structure and use as a dominant selectable marker for nuclear transformation

We have cloned and sequenced the CRY1 gene, encoding ribosomal protein S14 in Chlamydomonas reinhardtii, and found that it is highly similar to S14/rp59 proteins from other organisms, including mammals, Drosophila melanogaster, and Saccharomyces cerevisiae. We isolated a mutant strain resistant to the eukaryotic translational inhibitors cryptopleurine and emetine in which the resistance was due to a missense mutation (CRY1-1) in the CRY1 gene; resistance was dominant in heterozygous stable diploids. Cotransformation experiments using the CRY1-1 gene and the gene for nitrate reductase (NIT1) produced a low level of resistance to cryptopleurine and emetine. Resistance levels were increased when the CRY1-1 gene was placed under the control of a constitutive promoter from the ribulose bisphosphate carboxylase/oxygenase small subunit 2 (RBCS2) gene. We also found that the 5' untranslated region of the CRY1 gene was required for expression of the CRY1-1 transgene. Direct selection of emetine-resistant transformants was possible when transformed cells were first induced to differentiate into gametes by nitrogen starvation and then allowed to dedifferentiate back to vegetative cells before emetine selection was applied. With this transformation protocol, the RBCS2/CRY1-1 dominant selectable marker gene is a powerful tool for many molecular genetic applications in C. reinhardtii.

Introduction of exogenous DNA into Chlamydomonas reinhardtii by electroporation

The fate of exogenous DNA introduced into Chlamydomonas reinhardtii by electroporation was analyzed. With single and double electrical pulses, plasmids as large as 14 kb were introduced into cells with and without intact cell walls. Within hours after introduction, exogenous plasmid DNA was associated with nuclei isolated from cells; several weeks after introduction, exogenous DNA was stably integrated into the Chlamydomonas genome. These studies establish electroporation as a method for introducing DNA, and potentially other molecules, into C. reinhardtii.

Introduction of exogenous DNA into Chlamydomonas reinhardtii by electroporation

The fate of exogenous DNA introduced into Chlamydomonas reinhardtii by electroporation was analyzed. With single and double electrical pulses, plasmids as large as 14 kb were introduced into cells with and without intact cell walls. Within hours after introduction, exogenous plasmid DNA was associated with nuclei isolated from cells; several weeks after introduction, exogenous DNA was stably integrated into the Chlamydomonas genome. These studies establish electroporation as a method for introducing DNA, and potentially other molecules, into C. reinhardtii.

Stable nuclear transformation of Chlamydomonas reinhardtii by using a C. reinhardtii gene as the selectable marker

We have developed a stable nuclear transformation system for the unicellular green alga Chlamydomonas reinhardtii. Transformation was accomplished by introducing the cloned C. reinhardtii oxygen-evolving enhancer protein 1 (OEE1) gene into C. reinhardtii cells by bombardment with DNA-coated tungsten particles. The recipient strain was an OEE1-deficient, nonphotosynthetic, acetate-requiring mutant, which recovered photosynthetic competence after transformation, and was therefore able to grow in the absence of acetate. Analysis of several transformants indicates that transformation has proceeded via second-site integration of the cloned gene, leaving the endogenous mutant gene intact. In genetic crosses of transformants with wild type, both mutant and wild-type phenotypes were recovered, showing that the photosynthetic competence of transformants was due not to reversion of the original locus but rather to expression of the introduced gene. We suggest that the success of the present system is largely due to using a homologous C. reinhardtii gene, leading to stable maintenance and expression of the gene. Transformation with heterologous genes may be problematic because of poor expression due to an unusual codon bias in C. reinhardtii.

Stable nuclear transformation of Chlamydomonas using the Chlamydomonas gene for nitrate reductase

We have developed a nuclear transformation system for Chlamydomonas reinhardtii, using micro-projectile bombardment to introduce the gene encoding nitrate reductase into a nit1 mutant strain which lacks nitrate reductase activity. By using either supercoiled or linear plasmid DNA, transformants were recovered consistently at a low efficiency, on the order of 15 transformants per microgram of plasmid DNA. In all cases the transforming DNA was integrated into the nuclear genome, usually in multiple copies. Most of the introduced copies were genetically linked to each other, and they were unlinked to the original nit1 locus. The transforming DNA and nit+ phenotype were stable through mitosis and meiosis, even in the absence of selection. nit1 transcripts of various sizes were expressed at levels equal to or greater than those in wild-type nit+ strains. In most transformants, nitrate reductase enzyme activity was expressed at approximately wild-type levels. In all transformants, nit1 mRNA and nitrate reductase enzyme activity were repressed in cells grown on ammonium medium, showing that expression of the integrated nit1 genes was regulated normally. When a second plasmid with a nonselectable gene was bombarded into the cells along with the nit1 gene, transformants carrying DNA from both plasmids were recovered. In some cases, expression of the unselected gene could be detected. With the advent of nuclear transformation in Chlamydomonas, it becomes the first photosynthetic organism in which both the nuclear and chloroplast compartments can be transformed.

Structure and expression of the gene encoding the periplasmic arylsulfatase of Chlamydomonas reinhardtii

Chlamydomonas reinhardtii produces a periplasmic arylsulfatase in response to sulfur deprivation. We have isolated and sequenced arylsulfatase cDNAs from a lambda gt11 expression library. The amino acid sequence of the protein, as deduced from the nucleotide sequence, has features characteristic of secreted proteins, including a signal sequence and putative glycosylation sites. The gene has a broad codon usage with seven codons, all having A residues in the third position, not previously observed in C. reinhardtii genes. Arylsulfatase transcription is tightly regulated by sulfur availability. The approximately 2.7 kb arylsulfatase transcript is very susceptible to degradation, disappearing in less than an hour after sulfur starved cells are administered either sulfate or alpha-amanitin. The accumulation of the arylsulfatase transcript is also suppressed by the addition of cycloheximide. Transcription initiation from the arylsulfatase gene occurs approximately 100 bp upstream of the initiation codon, in a region that is 5' to a 43 bp imperfect inverted repeat. Preceding the transcription start site are sequences similar to those present in promoter regions of other genes from C. reinhardtii.

Molecular cloning and expression of flagellar radial spoke and dynein genes of Chlamydomonas

Several flagellar dynein ATPase and radial spokehead genes have been isolated from a Chlamydomonas genomic expression library in lambda gt11. The library was probed with polyclonal and monoclonal antibodies raised against purified flagellar polypeptides, and recombinant phage giving positive signals were cloned. In vitro translation of mRNAs hybrid-selected by the cloned sequences from whole cell RNA provided confirmation of identity for three of the four clones. Evidence supporting the identification of the fourth, which encodes a dynein heavy chain, was provided by antibody selection; the fusion protein produced by this clone selected heavy chain-specific antibodies from a complex polyclonal antiserum recognizing many dynein determinants. One of the radial spoke sequences isolated here is of particular interest because it encodes the wild-type allele of a locus which was defined previously by temperature-sensitive paralyzed flagella mutation pf-26ts (Huang, B., G. Piperno, Z. Ramanis, and D. J. L. Luck, 1981, J. Cell Biol., 88:80-88). The cloned sequence was used to hybrid-select mRNA from mutant pf-26ts cells, and when translated in vitro, the selected mRNA produced a mutant spokehead polypeptide with an altered electrophoretic mobility. This confirms that the pf-26ts mutation alters the primary structure of a radial spokehead polypeptide. To quantify spokehead and dynein mRNAs during flagellar regeneration, all of the cloned sequences were used as hybridization probes in RNA dot experiments. Levels increased rapidly and coordinately after deflagellation, peaked 3-10-fold above nondeflagellated controls, and then returned to control values within 2 h. This accumulation pattern was similar to that of flagellar alpha-tubulin mRNA.