Integrative Biology Department, University of South Florida, Tampa, Florida, USA.
Appl Environ Microbiol. 2024 Sep 18;90(9):e0107524. doi: 10.1128/aem.01075-24. Epub 2024 Aug 23.
Autotrophic bacteria are able to fix CO in a great diversity of habitats, even though this dissolved gas is relatively scarce at neutral pH and above. As many of these bacteria rely on CO fixation by ribulose 1,5-bisphospate carboxylase/oxygenase (RubisCO) for biomass generation, they must compensate for the catalytical constraints of this enzyme with CO-concentrating mechanisms (CCMs). CCMs consist of CO and HCO transporters and carboxysomes. Carboxysomes encapsulate RubisCO and carbonic anhydrase (CA) within a protein shell and are essential for the operation of a CCM in autotrophic that use the Calvin-Benson-Basham cycle. Members of the genus lack genes homologous to those encoding previously described CA, and prior to this work, the mechanism of function for their carboxysomes was unclear. In this paper, we provide evidence that a member of the recently discovered iota family of carbonic anhydrase enzymes (ιCA) plays a role in CO fixation by carboxysomes from members of and potentially other . Carboxysome enrichments from and were found to have CA activity and contain ιCA, which is encoded in their carboxysome loci. When the gene encoding ιCA was interrupted in , cells could no longer grow under low-CO conditions, and CA activity was no longer detectable in their carboxysomes. When ιCA was expressed in a strain of lacking native CA activity, this strain recovered an ability to grow under low CO conditions, and CA activity was present in crude cell extracts prepared from this strain.
Here, we provide evidence that iota carbonic anhydrase (ιCA) plays a role in CO fixation by some organisms with CO-concentrating mechanisms; this is the first time that ιCA has been detected in carboxysomes. While ιCA genes have been previously described in other members of bacteria, this is the first description of a physiological role for this type of carbonic anhydrase in this domain. Given its distribution in alkaliphilic autotrophic bacteria, ιCA may provide an advantage to organisms growing at high pH values and could be helpful for engineering autotrophic organisms to synthesize compounds of industrial interest under alkaline conditions.
自养细菌能够在多种多样的栖息地中固定 CO，尽管在中性 pH 值及以上时，这种溶解气体相对较少。由于这些细菌中的许多依赖于核酮糖 1,5-二磷酸羧化酶/加氧酶 (RubisCO) 固定 CO 来生成生物量，因此它们必须通过 CO 浓缩机制 (CCM) 来补偿该酶的催化限制。CCM 由 CO 和 HCO 转运蛋白和羧化体组成。羧化体将 RubisCO 和碳酸酐酶 (CA) 包裹在蛋白质壳内，对于使用卡尔文-本森-巴斯姆循环的自养生物的 CCM 运行是必不可少的。属的成员缺乏与先前描述的 CA 编码基因同源的基因，并且在这项工作之前，其羧化体的功能机制尚不清楚。在本文中，我们提供的证据表明，最近发现的碳酸酐酶酶 ι 家族的成员（ιCA）在属成员的羧化体固定 CO 中发挥作用，并且可能在其他属中也发挥作用。从和中富集的羧化体被发现具有 CA 活性并且包含 ιCA，其在它们的羧化体基因座中被编码。当编码 ιCA 的基因在中被中断时，细胞在低 CO 条件下无法再生长，并且它们的羧化体中不再检测到 CA 活性。当在缺乏天然 CA 活性的菌株中表达时，该菌株恢复了在低 CO 条件下生长的能力，并且从该菌株制备的粗细胞提取物中存在 CA 活性。
在这里，我们提供的证据表明，iota 碳酸酐酶（ιCA）在一些具有 CO 浓缩机制的生物体中固定 CO 中发挥作用；这是首次在羧化体中检测到 ιCA。虽然 ιCA 基因以前在其他细菌成员中已有描述，但这是首次在该领域描述这种类型的碳酸酐酶的生理作用。鉴于其在嗜碱自养细菌中的分布，ιCA 可能为在高 pH 值下生长的生物体提供优势，并且可能有助于对自养生物进行工程改造，以便在碱性条件下合成具有工业价值的化合物。
