Eukaryotic microalgae hold great promise for the bioproduction of fuels and higher value chemicals. However, compared with model genetic organisms such as Escherichia coli and Saccharomyces cerevisiae, characterization of the complex biology and biochemistry of algae and strain improvement has been hampered by the inefficient genetic tools. To date, many algal species are transformable only via particle bombardment, and the introduced DNA is integrated randomly into the nuclear genome. Here we describe the first nuclear episomal vector for diatoms and a plasmid delivery method via conjugation from Escherichia coli to the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana. We identify a yeast-derived sequence that enables stable episome replication in these diatoms even in the absence of antibiotic selection and show that episomes are maintained as closed circles at copy number equivalent to native chromosomes. This highly efficient genetic system facilitates high-throughput functional characterization of algal genes and accelerates molecular phytoplankton research.

Hematopoietic stem cells (HSCs) balance self-renewal and differentiation to maintain homeostasis. With aging, the frequency of polar HSCs decreases. Cell polarity in HSCs is controlled by the activity of the small RhoGTPase cell division control protein 42 (Cdc42). Here we demonstrate—using a comprehensive set of paired daughter cell analyses that include single-cell 3D confocal imaging, single-cell transplants, single-cell RNA-seq, and single-cell transposase-accessible chromatin sequencing (ATAC-seq)—that the outcome of HSC divisions is strongly linked to the polarity status before mitosis, which is in turn determined by the level of the activity Cdc42 in stem cells. Aged apolar HSCs undergo preferentially self-renewing symmetric divisions, resulting in daughter stem cells with reduced regenerative capacity and lymphoid potential, while young polar HSCs undergo preferentially asymmetric divisions. Mathematical modeling in combination with experimental data implies a mechanistic role of the asymmetric sorting of Cdc42 in determining the potential of daughter cells via epigenetic mechanisms. Therefore, molecules that control HSC polarity might serve as modulators of the mode of stem cell division regulating the potential of daughter cells.

Transfer of genomes into yeast facilitates genome engineering for genetically intractable organisms, but this process has been hampered by the need for cumbersome isolation of intact genomes before transfer. Here we demonstrate direct cell-to-cell transfer of bacterial genomes as large as 1.8 megabases (mb) into yeast under conditions that promote cell fusion. Moreover, we discovered that removal of restriction endonucleases from donor bacteria resulted in the enhancement of genome transfer.

Centromeres are essential for cell division and growth in all eukaryotes, and knowledge of their sequence and structure guides the development of artificial chromosomes for functional cellular biology studies. Centromeric proteins are conserved among eukaryotes; however, centromeric DNA sequences are highly variable. We combined forward and reverse genetic approaches with chromatin immunoprecipitation to identify centromeres of the model diatom Phaeodactylum tricornutum. We observed 25 unique centromere sequences typically occurring once per chromosome, a finding that helps to resolve nuclear genome organization and indicates monocentric regional centromeres. Diatom centromere sequences contain low-GC content regions but lack repeats or other conserved sequence features. Native and foreign sequences with similar GC content to P. tricornutum centromeres can maintain episomes and recruit the diatom centromeric histone protein CENH3, suggesting nonnative sequences can also function as diatom centromeres. Thus, simple sequence requirements may enable DNA from foreign sources to persist in the nucleus as extrachromosomal episomes, revealing a potential mechanism for organellar and foreign DNA acquisition.diatom | Phaeodactylum tricornutum | episome | centromere | CENH3

BackgroundSynthetic genomic approaches offer unique opportunities to use powerful yeast and Escherichia coli genetic systems to assemble and modify chromosome-sized molecules before returning the modified DNA to the target host. For example, the entire 1 Mb Mycoplasma mycoides chromosome can be stably maintained and manipulated in yeast before being transplanted back into recipient cells. We have previously demonstrated that cloning in yeast of large (> ~ 150 kb), high G + C (55%) prokaryotic DNA fragments was improved by addition of yeast replication origins every ~100 kb. Conversely, low G + C DNA is stable (up to at least 1.8 Mb) without adding supplemental yeast origins. It has not been previously tested whether addition of yeast replication origins similarly improves the yeast-based cloning of large (>150 kb) eukaryotic DNA with moderate G + C content. The model diatom Phaeodactylum tricornutum has an average G + C content of 48% and a 27.4 Mb genome sequence that has been assembled into chromosome-sized scaffolds making it an ideal test case for assembly and maintenance of eukaryotic chromosomes in yeast.ResultsWe present a modified chromosome assembly technique in which eukaryotic chromosomes as large as ~500 kb can be assembled from cloned ~100 kb fragments. We used this technique to clone fragments spanning P. tricornutum chromosomes 25 and 26 and to assemble these fragments into single, chromosome-sized molecules. We found that addition of yeast replication origins improved the cloning, assembly, and maintenance of the large chromosomes in yeast. Furthermore, purification of the fragments to be assembled by electroelution greatly increased assembly efficiency.ConclusionsEntire eukaryotic chromosomes can be successfully cloned, maintained, and manipulated in yeast. These results highlight the improvement in assembly and maintenance afforded by including yeast replication origins in eukaryotic DNA with moderate G + C content (48%). They also highlight the increased efficiency of assembly that can be achieved by purifying fragments before assembly.