Mol Plant Microbe Interact. 2014 May;27(5):437-45. doi: 10.1094/MPMI-11-13-0338-R.
Sinorhizobium meliloti, the nitrogen-fixing bacterial symbiont of Medicago spp. and other legumes, secretes a considerable amount of riboflavin. This precursor of the cofactors flavin mononucleotide and flavin adenine dinucleotide is a bioactive molecule that has a beneficial effect on plant growth. The ribBA gene of S. meliloti codes for a putative bifunctional enzyme with dihydroxybutanone phosphate synthase and guanosine triphosphate (GTP) cyclohydrolase II activities, catalyzing the initial steps of the riboflavin biosynthesis pathway. We show here that an in-frame deletion of ribBA does not cause riboflavin auxotrophy or affect the ability of S. meliloti to establish an effective symbiosis with the host plant but does affect the ability of the bacteria to secrete flavins, colonize host-plant roots, and compete for nodulation. A strain missing the RibBA protein retains considerable GTP cyclohydrolase II activity. Based on these results, we hypothesize that S. meliloti has two partly interchangeable modules for biosynthesis of riboflavin, one fulfilling the internal need for flavins in bacterial metabolism and the other producing riboflavin for secretion. Our data also indicate that bacteria-derived flavins play a role in communication between rhizobia and the legume host and that the RibBA protein is important in this communication process even though it is not essential for riboflavin biosynthesis and symbiosis.
根瘤农杆菌是紫花苜蓿和其他豆科植物的固氮细菌共生体，会分泌大量核黄素。这种黄素单核苷酸和黄素腺嘌呤二核苷酸辅因子的前体是一种具有生物活性的分子，对植物生长有有益的影响。根瘤农杆菌的 ribBA 基因编码一种假定的双功能酶，具有二羟丙酮磷酸合酶和鸟苷三磷酸 (GTP) 环化水解酶 II 的活性，催化核黄素生物合成途径的初始步骤。我们在这里表明，ribBA 的框内缺失不会导致核黄素营养缺陷或影响根瘤农杆菌与宿主植物建立有效共生的能力，但会影响细菌分泌黄素、定殖宿主植物根系和竞争结瘤的能力。缺失 RibBA 蛋白的菌株保留了相当大的 GTP 环化水解酶 II 活性。基于这些结果，我们假设根瘤农杆菌有两个部分可互换的模块用于核黄素的生物合成，一个满足细菌代谢中黄素的内部需求，另一个产生黄素用于分泌。我们的数据还表明，细菌衍生的黄素在根瘤菌和豆科宿主之间的通讯中发挥作用，并且即使对于核黄素生物合成和共生不是必需的，RibBA 蛋白在这个通讯过程中也很重要。
