Cytoduction in Chlamydomonas reinhardtii

The Spermidine Synthase Gene SPD1: A Novel Auxotrophic Marker for Chlamydomonas reinhardtii Designed by Enhanced CRISPR/Cas9 Gene Editing

Biotechnological application of the green microalga Chlamydomonas reinhardtii hinges on the availability of selectable markers for effective expression of multiple transgenes. However, biological safety concerns limit the establishment of new antibiotic resistance genes and until today, only a few auxotrophic markers exist for C. reinhardtii. The recent improvements in gene editing via CRISPR/Cas allow directed exploration of new endogenous selectable markers. Since editing frequencies remain comparably low, a Cas9-sgRNA ribonucleoprotein (RNP) delivery protocol was strategically optimized by applying nitrogen starvation to the pre-culture, which improved successful gene edits from 10% to 66% after pre-selection. Probing the essential polyamine biosynthesis pathway, the spermidine synthase gene (SPD1) is shown to be a potent selectable marker with versatile biotechnological applicability. Very low levels of spermidine (0.75 mg/L) were required to maintain normal mixotrophic and phototrophic growth in newly designed spermidine auxotrophic strains. Complementation of these strains with a synthetic SPD1 gene was achieved when the mature protein was expressed in the cytosol or targeted to the chloroplast. This work highlights the potential of new selectable markers for biotechnology as well as basic research and proposes an effective pipeline for the identification of new auxotrophies in C. reinhardtii.

Cell biology of yeast zygotes, from genesis to budding

The zygote is the essential intermediate that allows interchange of nuclear, mitochondrial and cytosolic determinants between cells. Zygote formation in Saccharomyces cerevisiae is accomplished by mechanisms that are not characteristic of mitotic cells. These include shifting the axis of growth away from classical cortical landmarks, dramatically reorganizing the cell cortex, remodeling the cell wall in preparation for cell fusion, fusing with an adjacent partner, accomplishing nuclear fusion, orchestrating two steps of septin morphogenesis that account for a delay in fusion of mitochondria, and implementing new norms for bud site selection. This essay emphasizes the sequence of dependent relationships that account for this progression from cell encounters through zygote budding. It briefly summarizes classical studies of signal transduction and polarity specification and then focuses on downstream events.

Degenerated recognition property of a mitochondrial homing enzyme in the unicellular green alga Chlamydomonas smithii

Target sequence cleavage is the essential step for intron invasion into an intronless allele. DNA cleavage at a specific site is performed by an endonuclease, termed a homing enzyme, which is encoded by an open reading frame within the intron. The recognition properties of them have only been analyzed in vitro, using purified, recombinant homing enzyme and various mutated DNA substrates, but it is unclear whether the homing enzyme behaves similarly in vivo. To answer this question, we determined the recognition properties of I-CsmI in vivo. I-CsmI is a homing enzyme encoded by the open reading frame of the alpha-group I-intron, located in the mitochondrial apocytochrome b gene of the green alga Chlamydomonas smithii. The in vivo recognition properties of it were determined as the frequency of intron invasion into a mutated target site. For this purpose, we utilized hybrid diploid cells developed by crossing alpha-intron-plus C. smithii to intron-minus C. reinhardtii containing mutated target sequences. The intron invasion frequency was much higher than the expected from the in vitro cleavage frequency of the respective mutated substrates. Even the substrates that had very little cleavage in the in vitro experiment were efficiently invaded in vivo, and were accompanied by a large degree of coconversion. Considering the ease of the homing enzyme invading into various mutated target sequences, we propose that the principle bottleneck for lateral intron transmission is not the sequence specificity of the homing enzyme, but instead is limited by the rare occurrence of inter-specific cell fusion.

Mutants of Chlamydomonas reinhardtii deficient in mitochondrial complex I: characterization of two mutations affecting the nd1 coding sequence

The mitochondrial rotenone-sensitive NADH:ubiquinone oxidoreductase (complex I) comprises more than 30 subunits, the majority of which are encoded by the nucleus. In Chlamydomonas reinhardtii, only five components of complex I are coded for by mitochondrial genes. Three mutants deprived of complex I activity and displaying slow growth in the dark were isolated after mutagenic treatment with acriflavine. A genetical analysis demonstrated that two mutations (dum20 and dum25) affect the mitochondrial genome whereas the third mutation (dn26) is of nuclear origin. Recombinational analyses showed that dum20 and dum25 are closely linked on the genetic map of the mitochondrial genome and could affect the nd1 gene. A sequencing analysis confirmed this conclusion: dum20 is a deletion of one T at codon 243 of nd1; dum25 corresponds to a 6-bp deletion that eliminates two amino acids located in a very conserved hydrophilic segment of the protein.

New tools for mitochondrial genetics of Chlamydomonas reinhardtii: manganese mutagenesis and cytoduction

A novel and efficient genetic procedure is described for generating mitochondrial mutants of the green alga Chlamydomonas reinhardtii. The development of a mutagenesis procedure using manganese cations and the application of cytoduction techniques resulted in a combined approach for the generation and analysis of mitochondrial mutants. Although mitochondrial mutations are inherited in sexual crosses from the minus mating type parent, the cytoduction technique can be used to transfer mitochondrial mutations into recipient strains with different genetic backgrounds, irrespective of their mating type. Cytoduction allows the transfer of mitochondrial markers from diploid to haploid cells also, which is of great benefit since diploid cells do not germinate in C. reinhardtii. We report here the isolation and characterisation of eight mutants, which are resistant to the antibiotics myxothiazol and mucidin. The mutants all have point mutations in the mitochondrial gene for apocytochrome b. Using in vitro-amplified cytb gene fragments as probes for direct DNA sequencing, three different types of single base pair substitutions were revealed in all mutants tested. In particular, amino acid substitutions in the mutant apocytochrome b polypeptide have been identified at residues 129, 132 and 137, which have been implicated in forming part of an antibiotic-binding niche. The amino acid substitution at position 132 has not been so far described for mutant apocytochrome b in any other organism, prokaryotic or eukaryotic. The genetic approach presented here confirms C. reinhardtii as a model system that is unique among plant cells.