Natural variation in preparation for nutrient depletion reveals a cost-benefit tradeoff

Maximizing growth and survival in the face of a complex, time-varying environment is a common problem for single-celled organisms in the wild. When offered two different sugars as carbon sources, microorganisms first consume the preferred sugar, then undergo a transient growth delay, the “diauxic lag,” while inducing genes to metabolize the less preferred sugar. This delay is commonly assumed to be an inevitable consequence of selection to maximize use of the preferred sugar. Contrary to this view, we found that many natural isolates of Saccharomyces cerevisiae display short or nonexistent diauxic lags when grown in mixtures of glucose (preferred) and galactose. These strains induce galactose utilization (GAL) genes hours before glucose exhaustion, thereby “preparing” for the transition from glucose to galactose metabolism. The extent of preparation varies across strains, and seems to be determined by the steady-state response of GAL genes to mixtures of glucose and galactose rather tha...在复杂且随时间动态变化的环境中实现生长与存活能力的最大化，是野外单细胞生物普遍面临的核心生存难题。当以两种不同糖类作为碳源时，微生物会优先消耗更适宜的糖类，随后经历一段短暂的生长停滞期——即二次生长延迟（diauxic lag）——同时诱导表达代谢次优糖类所需的基因。长期以来，这类生长停滞被认为是为最大化利用优先糖类而产生的进化选择的必然结果。但与这一主流观点相悖，我们发现，在葡萄糖（优先碳源）与半乳糖的混合培养基中培养时，诸多自然分离得到的酿酒酵母（Saccharomyces cerevisiae）菌株仅表现出极短甚至完全不存在的二次生长延迟。这些菌株会在葡萄糖耗尽前数小时就诱导表达半乳糖利用（GAL）基因，从而提前做好从葡萄糖代谢向半乳糖代谢转换的准备。不同菌株的提前准备程度存在差异，且这一差异似乎由GAL基因对葡萄糖与半乳糖混合底物的稳态响应所决定，而非……