Characterization of the Small RNA Transcriptome of the Marine Coccolithophorid, Emiliania huxleyi

MicroRNA-mediated regulation of lipid metabolism in virus-infected Emiliania huxleyi

The interactions between Emiliania huxleyi and E. huxleyi virus (EhV) regulate marine carbon and sulfur biogeochemical cycles and play a prominent role in global climate change. As a large DNA virus, EhV has developed a novel "virocell metabolism" model to meet its high metabolic needs. Although it has been widely demonstrated that EhV infection can profoundly rewire lipid metabolism, the epigenetic regulatory mechanisms of lipid metabolism are still obscure. MicroRNAs (miRNAs) can regulate biological pathways by targeting hub genes in the metabolic processes. In this study, the transcriptome, lipidome, and miRNAome were applied to investigate the epigenetic regulation of lipid metabolism in E. huxleyi cells during a detailed time course of viral infection. Combined transcriptomic, lipidomic, and physiological experiments revealed reprogrammed lipid metabolism, along with mitochondrial dysfunction and calcium influx through the cell membrane. A total of 69 host miRNAs (including 1 known miRNA) and 7 viral miRNAs were identified, 27 of which were differentially expressed. Bioinformatic prediction revealed that miRNAs involved in the regulation of lipid metabolism and a dual-luciferase reporter assay suggested that phosphatidylinositol 3-kinase (PI3K) gene might be a target of ehx-miR5. Further qPCR and western blot analysis showed a significant negative correlation between the expression of ehx-miR5 and its target gene PI3K, along with the lower activity of its downstream components (p-Akt, p-TOR, SREBP), indicating that lipid metabolism might be regulated by ehx-miR5 through the PI3K-Akt-TOR signaling pathway. Our findings reveal several novel mechanisms of viral strategies to manipulate host lipid metabolism and provide evidence that ehx-miR5 negatively modulates the expression of PI3K and disturbs lipid metabolism in the interactions between E. huxleyi and EhV.

Rampant nuclear-mitochondrial-plastid phylogenomic discordance in globally distributed calcifying microalgae

Incongruent phylogenies have been widely observed between nuclear and plastid or mitochondrial genomes in terrestrial plants and animals. However, few studies have examined these patterns in microalgae or the discordance between the two organelles. Here we investigated the nuclear-mitochondrial-plastid phylogenomic incongruence in Emiliania-Gephyrocapsa, a group of cosmopolitan calcifying phytoplankton with enormous populations and recent speciations. We assembled mitochondrial and plastid genomes of 27 strains from across global oceans and temperature regimes, and analyzed the phylogenomic histories of the three compartments using concatenation and coalescence methods. Six major clades with varying morphology and distribution are well recognized in the nuclear phylogeny, but such relationships are absent in the mitochondrial and plastid phylogenies, which also differ substantially from each other. The rampant phylogenomic discordance is due to a combination of organellar capture (introgression), organellar genome recombination, and incomplete lineage sorting of ancient polymorphic organellar genomes. Hybridization can lead to replacements of whole organellar genomes without introgression of nuclear genes and the two organelles are not inherited as a single cytoplasmic unit. This study illustrates the convoluted evolution and inheritance of organellar genomes in isogamous haplodiplontic microalgae and provides a window into the phylogenomic complexity of marine unicellular eukaryotes.

Identification of drought-responsive miRNAs in Hippophae tibetana using high-throughput sequencing

MicroRNAs (miRNAs) play an important role in abiotic stress response in plants. However, the total miRNA profiles (miRNome) and drought-responsive miRNAs in H. tibetana have not been identified. In this study, we present the first report on the miRNome profiles of H. tibetana by high-throughput sequencing technology. 116 known and 4 predicted novel miRNAs were all identified in six H. tibetana samples. Moreover, to reveal the drought-responsive miRNAs in H. tibetana, we compared the miRNA profiles of H. tibetana grown under water sufficiency and drought stress. The results showed that 39 known miRNAs were up-regulated, while 34 miRNAs were downregulated under drought stress. Moreover, the expression of two novel miRNAs (novel_mir_24 and novel_mir_87) showed notable changes in response to drought stress. The target genes of these differentially expressed miRNAs were mainly enriched in cellular process, metabolic process, cell part, and response to stimulus. The identified drought-responsive miRNAs might be used for improving drought tolerance in H. tibetana and other plateau plants.

Unmasking cellular response of a bloom-forming alga to viral infection by resolving expression profiles at a single-cell level

Infection by large dsDNA viruses can lead to a profound alteration of host transcriptome and metabolome in order to provide essential building blocks to support the high metabolic demand for viral assembly and egress. Host response to viral infection can typically lead to diverse phenotypic outcome that include shift in host life cycle and activation of anti-viral defense response. Nevertheless, there is a major bottleneck to discern between viral hijacking strategies and host defense responses when averaging bulk population response. Here we study the interaction between Emiliania huxleyi, a bloom-forming alga, and its specific virus (EhV), an ecologically important host-virus model system in the ocean. We quantified host and virus gene expression on a single-cell resolution during the course of infection, using automatic microfluidic setup that captures individual algal cells and multiplex quantitate PCR. We revealed high heterogeneity in viral gene expression among individual cells. Simultaneous measurements of expression profiles of host and virus genes at a single-cell level allowed mapping of infected cells into newly defined infection states and allowed detection specific host response in a subpopulation of infected cell which otherwise masked by the majority of the infected population. Intriguingly, resistant cells emerged during viral infection, showed unique expression profiles of metabolic genes which can provide the basis for discerning between viral resistant and susceptible cells within heterogeneous populations in the marine environment. We propose that resolving host-virus arms race at a single-cell level will provide important mechanistic insights into viral life cycles and will uncover host defense strategies.

Almost all of our current understanding of the molecular mechanisms that govern host-pathogen interactions in the ocean is derived from experiments carried out at the population level, neglecting any heterogeneity. Here we used a single cell approach to unmask the phenotypic heterogeneity produced within infected populations of the cosmopolitan bloom-forming alga Emiliania huxleyi by its specific lytic virus. We found high variability in expression of viral genes among individual cells. This heterogeneity was used to map cells into their infection state and allowed to uncover a yet unrecognized host response. We also provide evidence that variability in host metabolic states provided a sensitive tool to decipher between susceptible and resistant cells.

Global analysis of sRNA target genes in Mycoplasma hyopneumoniae

BACKGROUND

Small RNAs (sRNAs) are noncoding molecules that regulate different cellular activities in several bacteria. The role of sRNAs in gene expression regulation is poorly characterized in the etiological agent of porcine enzootic pneumonia Mycoplasma hyopneumoniae. We performed a global analysis of the sRNAs, sRNA target genes and regulatory elements previously identified in their genome and analyzed the expression of some sRNAs and their target genes by quantitative RT-PCR (qPCR) in three different culture conditions.

RESULTS

Seven of the 145 sRNA target genes are organized as monocistronic genes (mCs) while the other 138 sRNA target genes are organized into transcriptional units (TU). The identification of transcriptional regulatory elements (promoter motif, DNA repeat sequence or intrinsic terminator) was verified in 116 of the 145 sRNA target genes. Moreover, the 29 sRNA target genes without regulatory elements revealed the presence of at least one regulatory element in the boundaries of the TU or in other internal genes of the TU. We verified that 16 sRNAs showed differential expression, seven in heat shock condition and 14 in oxidative stress condition. Analysis of the differential expression of the sRNA target genes showed that the tested sRNAs possibly regulate gene expression. The sRNA target genes were up- or down-regulated possibly in response to sRNA only under oxidative stress condition. Moreover, the sRNA target genes are involved in diverse processes of the cell, some of which could be linked to transcription processes and cell homeostasis.

CONCLUSION

Our results indicate that bacterial sRNAs could regulate a number of targets with various outcomes, and different correlations between the levels of sRNA transcripts and their target gene mRNAs were found, which suggest that the regulation of gene expression via sRNAs may play an important role in mycoplasma.

Draft Genome Sequences of Four Bacterial Strains Isolated from a Polymicrobial Culture of Naked (N-Type) Emiliania huxleyi CCMP1516

Strains of Sulfitobacter spp., Erythrobacter sp., and Marinobacter sp. were isolated from a polymicrobial culture of the naked (N-type) haptophyte Emiliania huxleyi strain CCMP1516. The genomes encode genes for the production of phytohormones, vitamins, and the consumption of their hosts' metabolic by-products, suggesting symbiotic interactions within this polymicrobial culture.

Draft Genome Sequences of Seven Bacterial Strains Isolated from a Polymicrobial Culture of Coccolith-Bearing (C-Type) Emiliania huxleyi M217

Strains of Rhodobacteraceae, Sphingomonadales, Alteromonadales, and Bacteroidetes were isolated from a polymicrobial culture of the coccolith-forming (C-type) haptophyte Emiliania huxleyi strain M217. The genomes encode genes for the production of algal growth factors and the consumption of their hosts’ metabolic by-products, suggesting that the polymicrobial culture harbors many symbiotic interactions.