de Philip P, Batut J, Boistard P
Laboratoire de Biologie Moléculaire des Relations Plantes-Microorganismes, Centre National de la Recherche Scientifique-Institut National de la Recherche Agronomique, Castanet-Toulosan, France.
J Bacteriol. 1990 Aug;172(8):4255-62. doi: 10.1128/jb.172.8.4255-4262.1990.
In Rhizobium meliloti, nif and fix genes, involved in nitrogen fixation during symbiosis with alfalfa, are under the control of two transcriptional regulators encoded by nifA and fixK. Expression of nifA and fixK is under the control of FixL/J, a two-component regulatory system. We showed, using Escherichia coli as a heterologous host, that FixL/J controls nifA and fixK expression in response to microaerobiosis. Furthermore, expression of the sensor gene fixL and of the activator gene fixJ under the control of two different promoters allowed us to show that FixL mediates microaerobic induction of nifA when the level of FixJ is low and aerobic repression of nifA when the level of FixJ is high. Similarly, activation of fixK occurred in microaerobiosis when the FixJ level was low in the presence of FixL. In contrast to nifA, fixK expression was not affected by FixL in aerated cultures when the level of FixJ was high. We conclude that R. meliloti FixL senses oxygen in the heterologous host E. coli consistent with the microaerobic induction of nifA and fixK in R. meliloti and that nifA and fixK promoters are differentially activated by FixJ in response to the oxygen signal.
在苜蓿根瘤菌中，与紫花苜蓿共生期间参与固氮的nif和fix基因受nifA和fixK编码的两种转录调节因子的控制。nifA和fixK的表达受双组分调节系统FixL/J的控制。我们利用大肠杆菌作为异源宿主表明，FixL/J响应微需氧环境控制nifA和fixK的表达。此外，在两个不同启动子控制下的传感器基因fixL和激活基因fixJ的表达使我们能够表明，当FixJ水平较低时，FixL介导nifA的微需氧诱导，而当FixJ水平较高时，FixL介导nifA的好氧抑制。同样，当存在FixL且FixJ水平较低时，fixK在微需氧环境中被激活。与nifA不同，当FixJ水平较高时，在通气培养物中fixK的表达不受FixL的影响。我们得出结论，苜蓿根瘤菌FixL在异源宿主大肠杆菌中感知氧气，这与苜蓿根瘤菌中nifA和fixK的微需氧诱导一致，并且nifA和fixK启动子在响应氧气信号时被FixJ差异激活。
