Diverse ecosystems host microbial relationships that are stabilized by nutrient cross-feeding. Cross-feeding can involve metabolites that should hold value for the producer. Externalization of such communally valuable metabolites is often unexpected and difficult to predict. Previously, we discovered purine externalization by Rhodopseudomonas palustris by its ability to rescue an Escherichia coli purine auxotroph. Here we found that an E. coli purine auxotroph can stably coexist with R. palustris due to purine cross-feeding. We identified the cross-fed purine as adenine. Adenine was externalized by R. palustris under diverse growth conditions. Computational modeling suggested that adenine externalization occurs via diffusion across the cytoplasmic membrane. RNAseq analysis led us to hypothesize that adenine accumulation and externalization stems from a salvage pathway bottleneck at the enzyme encoded by apt. Ectopic expression of apt eliminated adenine externalization, supporting our hypothesis. A comparison of 49 R. palustris strains suggested that purine externalization is relatively common, with 16 strains exhibiting the trait. Purine externalization was correlated with the genomic orientation of apt, but apt orientation alone could not always explain purine externalization. Our results provide a mechanistic understanding of how a communally valuable metabolite can participate in cross-feeding. Our findings also highlight the challenge in identifying genetic signatures for metabolite externalization.

中文翻译：

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细菌腺嘌呤交叉喂养源于嘌呤回收瓶颈

多样化的生态系统拥有通过营养物交叉取食而稳定的微生物关系。交叉喂养可能涉及对生产者有价值的代谢物。这种具有公共价值的代谢物的外化通常是意想不到的并且难以预测。此前，我们通过沼泽红假单胞菌拯救大肠杆菌嘌呤营养缺陷型的能力发现了嘌呤外化。在这里，我们发现，由于嘌呤交叉喂养，大肠杆菌嘌呤营养缺陷型可以与沼泽红菌稳定共存。我们将交叉喂养的嘌呤确定为腺嘌呤。腺嘌呤被沼泽红藻在不同的生长条件下外化。计算模型表明腺嘌呤外化是通过跨细胞质膜的扩散发生的。RNAseq 分析使我们假设腺嘌呤积累和外化源于 apt 编码的酶的补救途径瓶颈。apt 的异位表达消除了腺嘌呤外化，支持了我们的假设。对 49 个沼泽沼泽菌菌株的比较表明，嘌呤外化相对常见，其中 16 个菌株表现出这种特征。嘌呤外化与 apt 的基因组方向相关，但仅 apt 方向不能总是解释嘌呤外化。我们的结果提供了对具有共同价值的代谢物如何参与交叉喂养的机制理解。我们的研究结果还强调了识别代谢物外化遗传特征的挑战。