It is known that adaptive evolution in permanently cold environments drives cold adaptation in enzymes. However, how the relatively high enzyme activities were achieved in cold environments prior to cold adaptation of enzymes is unclear. Here we report that an Antarctic strain of Chlorella vulgaris, called NJ-7, acquired the capability to grow at near 0 °C temperatures and greatly enhanced freezing tolerance after systematic increases in abundance of enzymes/proteins and positive selection of certain genes. Having diverged from the temperate strain UTEX259 of the same species 2.5 (1.1-4.1) to 2.6 (1.0-4.5) Ma, NJ-7 retained the basic mesophilic characteristics and genome structures. Nitrate reductases in the two strains are highly similar in amino acid sequence and optimal temperature, but the NJ-7 one showed significantly higher abundance and activity. Quantitative proteomic analyses indicated that several cryoprotective proteins (LEA), many enzymes involved in carbon metabolism and a large number of other enzymes/proteins, were more abundant in NJ-7 than in UTEX259. Like nitrate reductase, most of these enzymes were not upregulated in response to cold stress. Thus, compensation of low specific activities by increased enzyme abundance appears to be an important strategy for early stage cold adaptation to Antarctica, but such enzymes are mostly not involved in cold acclimation upon transfer from favorable temperatures to near 0 °C temperatures.

中文翻译：

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中温绿藻对南极洲的早期适应：酶和LEA蛋白质丰度的系统性增加。

众所周知，在永久性寒冷环境中的适应性进化推动了酶的低温适应性。然而，尚不清楚如何在寒冷的环境中在酶的冷适应之前实现相对较高的酶活性。在这里我们报告说，南极小球藻菌株NJ-7具有在0°C附近生长的能力，并在系统增加酶/蛋白质丰度和某些基因的阳性选择后大大增强了冰冻耐受性。NJ-7从同一物种2.5（1.1-4.1）的温带菌株UTEX259分化为2.6（1.0-4.5）Ma的温带菌株后，保留了基本的嗜温特性和基因组结构。这两个菌株中的硝酸还原酶在氨基酸序列和最佳温度上高度相似，但NJ-7的丰度和活性明显更高。定量蛋白质组学分析表明，NJ-7中的几种冷冻保护蛋白（LEA），许多参与碳代谢的酶以及许多其他酶/蛋白比UTEX259丰富。像硝酸盐还原酶一样，大多数这些酶在冷胁迫下均未上调。因此，通过增加酶的丰度来补偿低比活性似乎是早期冷适应南极洲的重要策略，但是当从有利温度转移到接近0°​​C的温度时，此类酶大多不参与冷驯化。在NJ-7中比在UTEX259中更丰富。像硝酸盐还原酶一样，大多数这些酶在冷胁迫下均未上调。因此，通过增加酶的丰度来补偿低比活性似乎是早期冷适应南极洲的重要策略，但是当从有利温度转移到接近0°​​C的温度时，此类酶大多不参与冷驯化。在NJ-7中比在UTEX259中更丰富。像硝酸盐还原酶一样，大多数这些酶在冷胁迫下均未上调。因此，通过增加酶的丰度来补偿低比活性似乎是早期冷适应南极洲的重要策略，但是当从有利温度转移到接近0°​​C的温度时，此类酶大多不参与冷驯化。