The purple nonsulfur bacterium Rhodopseudomonas palustris is a model for understanding how a phototrophic organism adapts to changes in light intensity because it produces different light-harvesting (LH) complexes under high light (LH2) and low light intensities (LH3 and LH4). Outside of this change in the composition of the photosystem, little is understood about how R. palustris senses and responds to low light intensity. On the basis of the results of transcription analysis of 17 R. palustris strains grown in low light, we found that R. palustris strains downregulate many genes involved in iron transport and homeostasis. The only operon upregulated in the majority of R. palustris exposed to low light intensity was pucBAd, which encodes LH4. In previous work, pucBAd expression was shown to be modulated in response to light quality by bacteriophytochromes that are part of a low-light signal transduction system. Here we found that this signal transduction system also includes a redox-sensitive protein, LhfE, and that its redox sensitivity is required for LH4 synthesis in response to low light. Our results suggest that R. palustris upregulates its LH4 system when the cellular redox state is relatively oxidized. Consistent with this, we found that LH4 synthesis was upregulated under high light intensity when R. palustris was grown semiaerobically or under nitrogen-fixing conditions. Thus, changes in the LH4 system in R. palustris are not dependent on light intensity per se but rather on cellular redox changes that occur as a consequence of changes in light intensity.IMPORTANCE An essential aspect of the physiology of phototrophic bacteria is their ability to adjust the amount and composition of their light-harvesting apparatus in response to changing environmental conditions. The phototrophic purple bacterium R. palustris adapts its photosystem to a range of light intensities by altering the amount and composition of its peripheral LH complexes. Here we found that R. palustris regulates its LH4 complex in response to the cellular redox state rather than in response to light intensity per se Relatively oxidizing conditions, including low light, semiaerobic growth, and growth under nitrogen-fixing conditions, all stimulated a signal transduction system to activate LH4 expression. By understanding how LH composition is regulated in R. palustris, we will gain insight into how and why a photosynthetic organism senses and adapts its photosystem to multiple environmental cues.

中文翻译：

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产氧光养动物中光收集天线复合物的氧化还原调节。

紫色无硫细菌紫假单胞菌是理解光养生物如何适应光强度变化的模型，因为它在高光（LH2）和低光强度（LH3和LH4）下会产生不同的光收集（LH）复合物。除了光系统组成的这种变化之外，人们对帕氏疟原虫如何感测和响应低光强度的了解很少。根据在弱光下生长的17株R. palustris菌株的转录分析结果，我们发现R. palustris菌株下调了许多与铁运输和体内稳态有关的基因。在暴露于低光强度的大多数帕氏疟原虫中，唯一被上调的操纵子是pucBAd，它编码LH4。在以前的工作中，pucBAd的表达被证明是作为低光信号转导系统一部分的细菌植物色素对光质量的响应。在这里，我们发现该信号转导系统还包含氧化还原敏感蛋白LhfE，并且其氧化还原敏感度是LH4合成对低照度响应所必需的。我们的结果表明，当细胞氧化还原状态被相对氧化时，帕氏疟原虫会上调其LH4系统。与此相符，我们发现当高厌氧菌在半厌氧条件下或在固氮条件下生长时，LH4合成在高光强度下上调。因此，帕氏疟原虫的LH4系统的变化本身并不取决于光强度，而是取决于由于光强度变化而发生的细胞氧化还原变化。重要说明光养细菌生理学的一个重要方面是其能够响应不断变化的环境条件来调整其光收集设备的数量和组成的能力。紫色的光养细菌R. palustris通过改变其周围LH复合物的数量和组成，使其光系统适应一定的光强度。在这里，我们发现R. palustris调节细胞的氧化还原状态而不是响应光强度本身来调节其LH4复合物相对氧化的条件（包括低光照，半有氧生长和固氮条件下的生长）均刺激了信号转导系统激活LH4表达。通过了解palustris中LH的组成如何受到调控，