Klug Gabriele
Institute of Microbiology and Molecular Biology, IFZ, Justus Liebig University of Giessen, Heinrich-Buff-Ring 26-32, 35392, Giessen, Germany.
Mol Microbiol. 2014 Feb;91(4):635-40. doi: 10.1111/mmi.12490. Epub 2014 Jan 9.
Vitamin B12 is well known as an enzyme cofactor in the catalysis of many important biological reactions, and the role of B12 in regulation of bacterial gene expression as a ligand of riboswitches is well established. Only recently evidence has emerged that B12 can also affect bacterial gene expression by acting as a cofactor of regulatory proteins. In 2011 a role of B12 as a cofactor of the transcriptional repressor of carotenogenesis, CarH, in Myxococcus xanthus was reported. B12 is required for light-dependent DNA binding by CarH, which can therefore be considered to be a new type of photoreceptor. Cheng et al. (2014) report the identification of B12 as a cofactor of the AerR protein in Rhodobacter capsulatus. AerR acts as an antirepressor of the CrtJ protein, which represses photosynthesis genes when binding to its target promoters. As in Myxococcus B12 may have the role of a chromophore in photoreception, but it is suggested that a main function of AerR is the sensing of B12. The co-regulation of the pathways is beneficial because the syntheses of B12 , haem and bacteriochlorophylls share common precursors and the accumulation of the free molecules is toxic.
维生素B12作为许多重要生物反应催化过程中的一种酶辅因子广为人知，并且维生素B12作为核糖开关的配体在细菌基因表达调控中的作用也已得到充分证实。直到最近，有证据表明维生素B12还可以作为调节蛋白的辅因子来影响细菌基因表达。2011年，有报道称维生素B12在黄色粘球菌中作为类胡萝卜素生成转录阻遏物CarH的辅因子发挥作用。维生素B12是CarH依赖光进行DNA结合所必需的，因此可以认为CarH是一种新型光感受器。程等人（2014年）报道了在荚膜红细菌中维生素B12作为AerR蛋白辅因子的鉴定。AerR作为CrtJ蛋白的抗阻遏物发挥作用，CrtJ蛋白在与其靶启动子结合时会抑制光合作用基因。与黄色粘球菌的情况一样，维生素B12可能在光感受中起生色团的作用，但有人认为AerR的主要功能是感知维生素B12。这些途径的共同调节是有益的，因为维生素B12、血红素和细菌叶绿素的合成共享共同的前体，并且游离分子的积累是有毒的。
