Project description:Purpose: We aimed at i) obtaining insight into how a thermophile organism reacts to cold stress, and ii) evaluating the impact of HGT candidates on the acclimation process to temperature decrease. Methods: The experimental design followed a temperature shift timeline: after two weeks of cultivation at 42°C, constant illumination (90 μE) and constant shaking (160 rpm) in photoautotrophic conditions the first sampling took place (Hot_T48_1) and the cultures of G.sulphuraria were swiftly moved to 28°C ( = cold temperature). "Cold" samples were taken after 3h (Cold_T3_2), 12h (Cold_T12_3) and 48h (Cold_T48_4). After cold treatment at 28°C for 48 hours, the G. sulphuraria was then switched to 46°C for 48 hours (="Hot"). Again, samples were taken after 3h (Hot_T3_5), 12h (Hot_T12_6) and 48h (Hot_T48_7). Altogether, a 48h temperature timeshift at 28°C and successive recovery at 46°C were targeted for sampling. Results: Galdieria sulphuraria is a unicellular red alga that lives in hot, acidic, toxic metal-rich, volcanic environments, where few other organisms survive. Its genome harbours up to 5% of genes most likely acquired through horizontal gene transfer. These genes probably contributed to G. sulphuraria’s adaptation to its extreme habitats, resulting in today’s polyextremophilic traits. Here, we applied RNA-sequencing to obtain insights into the acclimation of a thermophilic organism towards temperatures below its growth optimum and to study how horizontally acquired genes contribute to cold acclimation. A decrease in growth temperature from 42 °C/46 °C to 28 °C resulted in an upregulation of ribosome biosynthesis, while excreted proteins, probably components of the cell wall, were downregulated. Photosynthesis was suppressed at cold temperatures, and transcript abundances indicated that C-metabolism switched from gluconeogenesis to glycogen degradation. Folate cycle and S-adenosylmethionine cycle (one-carbon metabolism) were transcriptionally upregulated, probably to drive the biosynthesis of betaine. All these cold-induced changes in gene expression were reversible upon temperature increase. Numerous genes acquired by horizontal gene transfer displayed pronounced temperature-dependent expression changes, corroborating the view that these genes contributed to adaptive evolution in G. sulphuraria.

Project description:Purpose: Red algae show two putative phases of massive genome reduction. RNA-splicing represents an important form of post-transcriptional regulation in eukaryotes. This process generates increased transcriptome and proteome diversity via alternative splicing that can maximize the DNA-encoded genetic diversity embedded within genomes. Why would Spliceosomal Machinery genes and a relatively more complex intron-exon structure be preserved in the compact Galdieria genome? The answer may lie in the fact that RNA-splicing represents an important form of post-transcriptional regulation in eukaryotes. It is hypothesized that his process generates increased transcriptome and proteome diversity via alternative splicing that can maximize the DNA-encoded genetic diversity embedded within genomes. Methods: The experimental design followed a temperature shift timeline: after two weeks of cultivation at 42°C, constant illumination (90 μE) and constant shaking (160 rpm) in photoautotrophic conditions the first sampling took place (H-42) and the cultures of G.sulphuraria were swiftly moved to 28°C. After cold treatment at 28°C for 48 hours, a second sampling was performed (C-28.1). The G. sulphuraria was then switched to 46°C for 48 hours, at end of which a third sampling was retrieved (H-46). It again was followed by a cold treatment at 28°C for 48 hours when a fourth sampling was retrieved (C-28.2). Altogether, two 48h timepoints from high temperatures (42°C and 46oC) and two 48h timepoints from cold temperature (28°C) were targeted for sampling. Results: We show that RNA-splicing provides an avenue for generating transcriptome diversity under strong constraints on genome size and gene number. Conclusions: We report the relatively high retention of Spliceosomal Machinery genes and the enrichment of intron-exon structures in G. sulphuraria that might be associated with the exceptional transcriptome and proteome diversity of this species. These results suggest that the losses of genes and functions caused by genome reduction might be mitigated by sequence diversity generated by a robust RNA-splicing system. More generally, the strong constraints imposed by genome reduction appear to be ameliorated by processes that operate on the transcriptomic and proteomic levels.

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Galdieria sulphuraria

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