Proper regulation of gene expression during cell cycle entry ensures the successful completion of proliferation, avoiding risks such as carcinogenesis. The microRNA (miRNA) network is an emerging molecular system regulating multiple genetic pathways. We demonstrate here that the global elevation of miRNAs is critical for proper control of gene expression program during cell cycle entry. Strikingly, Exportin 5 (XPO5) is promptly induced during cell cycle entry by a PI3K-dependent post-transcriptional mechanism. Inhibition of XPO5 induction interfered with global miRNA elevation and resulted in a proliferation defect associated with delayed G1/S transition. During cell cycle entry, XPO5 therefore plays a paramount role as a critical molecular hub controlling the gene expression program through global regulation of miRNAs. Our data suggest that XPO5-mediated global miRNA elevation might be involved in a broad range of cellular events associated with cell cycle control.
<p>Forest fires are an important factor in the global carbon cycle and high latitude ecosystems. Eastern Siberian tundra, summergreen larch-dominated boreal forest on permafrost and evergreen boreal forest have characteristic fire regimes with varying fire intensities. Yet, it is unknown which role fire plays in long-term climate-vegetation-permafrost feedbacks and how high-latitude fire regimes and ecosystems will change in a warmer world. To learn from fire regime shifts during previous interglacials, prior to human presence, we use lake-sedimentary charcoal as proxy for high-intensity forest fires and monosaccharide anhydrides (i.e. levoglucosan, mannosan, galactosan: MA) as molecular proxies for low-temperature biomass burning, typical for surface fires in modern larch forest. However, MA pathways from source to sink and their stability in sediments are very poorly constrained. Recently, Dietze et al. (2020) found MA in up to 420 kyr old sediment of Lake El&#8217;gygytgyn (ICDP Site 5011-1), NE Siberia, suggesting that they are suitable proxies for fires in summergreen boreal forests. Surprisingly, the ratios of the MA isomers were exceptionally low compared to published emission ratios from modern combustions.</p><p>To understand what MA from Arctic lake sediments tell us, we have analyzed the MA and charcoal composition in modern lake surface sediments of Lake El&#8217;gygytgyn and three East Siberian lakes and we compare them to late glacial-to-interglacial El&#8217;gygytgyn records. The three Siberian lakes were chosen to represent spatial analogues to the El&#8217;gygytgyn conditions during MIS 5e and 11c. We discuss first results of the modern sediments in context of recent MODIS- and Landsat-based fire extents and biome-specific land cover data, a wind field modelling using climate data over eastern Siberia, and lake-catchment configurations from TDX-DEM analysis to assess potential fire proxy source areas and regional-to-local transport processes. Thereby, we provide insights into the meaning of sedimentary fire proxies, crucial for a sound reconstruction of long-term fire regime histories.</p>
<p>The ongoing intensification of forest fires in the Siberian Arctic (larger areas, longer durations, higher intensities) raises concerns if these fires might lead to biome shifts from tundra to summergreen or evergreen boreal forest &#8211; with consequences for regional to global biophysical land properties and biogeochemical cycles. Given the short time span of instrumental observations, it is unknown if fire can initiate or support biome shifts under the ongoing amplified warming or if climate drives fire regime and biome changes independently. Lake El&#8217;gygytgyn in the Russian Far East is currently surrounded by tundra, but pollen data (ICDP sediment core 5011-1A) suggests that during late Marine Isotope Stage (MIS) 12 and &#8220;superinterglacial&#8221; MIS 11, c. 375-440 kyrs ago biomes changed several times: &#160;from a glacial steppe to interglacial summergreen and evergreen boreal forest. Here, we investigate if and which type of fire regime shifts accompanied these biome shifts.</p><p>To enable a quantitative reconstruction of changes in fire intensities and the type of biomass burnt, we used multiple fire proxies. The monosaccharide anhydrides (MAs) are specific biomass burning residues from low-temperature fires analyzed with ultra-high-performance liquid chromatography coupled to a high-resolution mass spectrometer. Sedimentary charcoal reflects mid-to-high intensity fires and was analyzed in sieved fraction > 150 &#956;m and from pollen slides using a microscope. MA isomer ratios and charcoal morphotypes were used to reconstruct the type of biomass burnt. We established links between fire proxy composition and pollen-based vegetation composition for the MIS 11 using core 5011-1A sediments considering dating uncertainties.</p><p>To link fire proxy composition with fire regime properties, we used the same fire proxies in modern lake surface sediments from three lakes in Eastern Siberia. We assessed modern charcoal and MA source areas by modelling lake-sedimentary fire proxy transport from modern fires using fire radiative power data of the MODIS Thermal Anomalies product, plume injection heights from the MODIS Terra and Aqua MCD19A2 data and hourly wind fields from ERA5 climate data.</p><p>We find clear differences in sedimentary fire proxy composition depending on source area of charcoals and MAs in modern lake sediments. Modern types of fire regime-fire proxy-vegetation-relationships are linkable to the derived past interglacial relationships indicating that fire regime change played a role during some, but not all biome shifts. Overall, we provide new understanding of Siberian sedimentary fire proxies, crucial for a sound, i.e. quantitative reconstruction of long-term fire regime change, allowing to assess the role of fire regime intensification in biome changes during periods of stark warming. </p>
