Jeong, B.-r. | bioRxiv Science

bioRxiv2021

Aureochromes are necessary for maintaining polyunsaturated fatty acid content in Nannochloropsis oceanica

Nannochloropsis oceanica, as other stramenopile microalgae, is rich in long-chain polyunsaturated fatty acids (LC-PUFA) such as eiconsapentaenoic acid (EPA). We observed that fatty acid desaturases (FAD) involved in LC-PUFA biosynthesis were among the strongest blue light induced genes in N. oceanica CCMP1779. Blue light was also necessary for maintaining LC-PUFA levels in CCMP1779 cells, and growth under red light led to a reduction in EPA content. Aureochromes are stramenopile specific proteins that contain a light-oxygen-voltage-sensing (LOV) domain that associates with a flavin mononucleotide and is able to sense blue light. These proteins also contain a bZIP DNA binding motif and can act as blue light regulated transcription factors by associating with a E-box like motif, which we found enriched in the promoters of blue light induced genes. We demonstrated that, in vitro, two CCMP1779 aureochromes were able to absorb blue light. Moreover, the loss or reduction of any of the three aureochromes led to a decrease in the blue light specific induction of several FADs in CCMP1779. EPA content was also significantly reduced in NoAureo 2 and NoAureo 4 mutants. Taken together, our results indicate that aureochromes mediate blue light dependent regulation of LC-PUFA content in N. oceanica CCMP1779 cells.

plant biology↗

bioRxiv2020

Genome engineering of Nannochloropsis with large deletions for constructing microalgal minigenomes

Industrial microalgae are promising photosynthetic cell factories, yet tools for targeted genome engineering are limited. Here for the model industrial oleaginous microalga Nannochloropsis oceanica we established a method to precisely and serially delete large genome fragments of ~100 kb from its 30.01-Mb nuclear genome. We started by identifying the "non-essential" chromosomal regions (i.e., low-expression region or LER) based on minimal gene expression under N-replete and N-depleted conditions. The largest such LER (LER1) is ~98 kb in size, located near the telomere of the 502.09 kb-long Chromosome 30 (Chr 30). We deleted 81 kb and further distal and proximal deletions of up to 110 kb (21.9% of Chr 30) in LER1 by dual targeting the boundaries with the episome-based CRISPR/Cas9 system. The telomere-deletion mutants showed normal telomeres consisting of CCCTAA repeats, revealing telomere regeneration capability after losing distal part of Chr 30. Interestingly, the deletions caused no significant alteration in growth, lipid production or photosynthesis (transcript-abundance change for < 3% genes under N depletion). We also performed double-deletion of both LER1 and LER2 (from Chr 9) that totals ~214 kb, and phenotypes are essentially normal. Therefore, loss of the large yet "non-essential" regions does not necessarily sacrifice important traits. Such serial targeted deletions of large genomic regions have not been reported in plants or microalgae, and will accelerate crafting minimal genomes as chassis for photosynthetic production.

biochemistry↗