Wells Derek H, Chen Esther J, Fisher Robert F, Long Sharon R
Department of Medicine, Division of Infectious Diseases, University of California, 513 Parnassus Avenue, San Francisco, CA 94143-0654, USA.
Mol Microbiol. 2007 May;64(3):647-64. doi: 10.1111/j.1365-2958.2007.05680.x.
Sinorhizobium meliloti enters into a symbiotic relationship with legume host plants, providing fixed nitrogen in exchange for carbon and amino acids. In S. meliloti, exoR and the exoS-chvI two-component system regulate the biosynthesis of succinoglycan, an exopolysaccharide important for host invasion. It was previously reported that a loss-of-function mutation in exoR and a gain-of-function mutation in exoS cause overproduction of succinoglycan and loss of motility, indicating that ExoR negatively regulates and ExoS-ChvI positively regulates downstream genes. However, a relationship between exoR and exoS-chvI has never been clearly established. By identification and detailed characterization of suppressor strains, we provide genetic evidence that exoR and exoS-chvI control many similar phenotypes. These include succinoglycan production, symbiosis, motility, and previously uncharacterized prototrophy and biofilm formation, all of which are co-ordinately restored by suppressors. We further demonstrate that ExoR is located in the periplasm, suggesting that it functions to regulate downstream genes in a novel manner. In pathogenic bacteria closely related to S. meliloti, exoS-chvI homologues are required for virulence and the regulation of cell envelope composition. Our data suggest that periplasmically localized ExoR and ExoS-ChvI function together in a unique and critical regulatory system associated with both free-living and symbiotic states of S. meliloti.
苜蓿中华根瘤菌与豆科宿主植物建立共生关系，提供固定氮以换取碳和氨基酸。在苜蓿中华根瘤菌中，exoR和exoS-chvI双组分系统调节琥珀聚糖的生物合成，琥珀聚糖是一种对宿主侵染很重要的胞外多糖。先前有报道称，exoR的功能缺失突变和exoS的功能获得突变会导致琥珀聚糖过量产生和运动性丧失，这表明ExoR负向调节，而ExoS-ChvI正向调节下游基因。然而，exoR和exoS-chvI之间的关系从未得到明确确立。通过对抑制菌株的鉴定和详细表征，我们提供了遗传学证据，证明exoR和exoS-chvI控制许多相似的表型。这些表型包括琥珀聚糖的产生、共生、运动性，以及以前未表征的原养型和生物膜形成，所有这些都通过抑制子得到协调恢复。我们进一步证明ExoR位于周质中，这表明它以一种新的方式调节下游基因。在与苜蓿中华根瘤菌密切相关的病原菌中，exoS-chvI同源物对于毒力和细胞包膜组成的调节是必需的。我们的数据表明，位于周质中的ExoR和ExoS-ChvI在一个与苜蓿中华根瘤菌的自由生活和共生状态相关的独特而关键的调节系统中共同发挥作用。
