Abstract
Results from previous experiments indicated that the Gram-negative α-proteobacterium Serratia liquefaciens strain ATCC 27592 was capable of growth under low temperature (0 °C), low pressure (0.7 kPa), and anoxic, CO2-dominated atmosphere–conditions intended to simulate the near-subsurface environment of Mars. To probe the response of its transcriptome to this extreme environment, S. liquefaciens ATCC 27592 was cultivated under 4 different environmental simulations: 0 °C, 0.7 kPa, CO2 atmosphere (Condition A); 0 °C, ~101.3 kPa, CO2 atmosphere (Condition B); 0 °C, ~101.3 kPa, ambient N2/O2 atmosphere (Condition C); and 30 °C, ~101.3 kPa, N2/O2 atmosphere (Condition D; ambient laboratory conditions). RNA-seq was performed on ribosomal RNA-depleted total RNA isolated from triplicate cultures grown under Conditions A-D and the datasets generated were subjected to transcriptome analyses. The data from Conditions A, B, or C were compared to laboratory Condition D. Significantly differentially expressed transcripts were identified belonging to a number of KEGG pathway categories. Up-regulated genes under all Conditions A, B, and C included those encoding transporters (ABC and PTS transporters); genes involved in translation (ribosomes and their biogenesis, biosynthesis of both tRNAs and aminoacyl-tRNAs); DNA repair and recombination; and non-coding RNAs. Genes down-regulated under all Conditions A, B, and C included: transporters (mostly ABC transporters); flagellar and motility proteins; genes involved in phenylalanine metabolism; transcription factors; and two-component systems. The results are discussed in the context of Mars astrobiology and planetary protection.
Highlights
A central goal of Astrobiology is to understand the potential for habitability in the universe, including determination of the physical limits at which life can exist and the mechanisms used by living organisms to survive and grow in extreme environments[1,2]
Subsequent studies testing the ability of various bacteria to grow under increasingly Mars-like laboratory simulations revealed that growth of most of the microbes tested, mostly strains obtained from laboratory collections or spacecraft assembly facilities, was inhibited by low temperature, low pressure, and anoxic, predominantly CO2 atmospheres, applied either singly or in pairwise combination[13,14]
Most bacteria were unable to grow under the conditions applied in these early experiments, recently a small but growing subset of bacterial species have been discovered that can grow under simultaneously applied conditions of low temperature (0 °C), low pressure (0.7 kPa) and atmospheric gas composition intended to simulate the physical conditions in the near subsurface of Mars
Summary
Characterization of the S. liquefaciens transcriptomic response to different physical environments. The comparative analysis identified 493 differentially expressed genes in Condition A (193 up- and 300 down-regulated); 708 genes in condition B (209 up- and 499 down-regulated) and 429 genes in Condition C (153 up- and 276 down-regulated) with respect to the reference Condition D Inspection of the data revealed that exposure of S. liquefaciens to Conditions A, B, or C each produced its own set of transcriptome responses. These sets were partially unique and partially overlapping in any combination examined (Fig. 2). A common set of 34 up-regulated and 156
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