Reyrat J M, David M, Batut J, Boistard P
Laboratoire de Biologie Moléculaire des Relations Plantes-Microorganismes, CNRS-INRA, BP27 31326 Castanet-Tolosan, France.
J Bacteriol. 1994 Apr;176(7):1969-76. doi: 10.1128/jb.176.7.1969-1976.1994.
In Rhizobium meliloti, transcription of nitrogen fixation genes is induced in oxygen-depleted conditions under the control of the two-component regulatory system FixLJ. FixJ is a transcriptional activator whose activity is dramatically enhanced by phosphorylation, whereas FixL is a hemoprotein kinase that controls the level of phosphorylated FixJ in response to oxygen availability. We have found that a mutant FixJ protein, FixJD54N, in which the presumed site of phosphorylation (aspartate 54) was changed to an asparagine, is strongly affected for phosphorylation by FixL and is not detectably phosphorylated from the low-molecular-weight phosphate donor, acetyl-phosphate. Unexpectedly, FixL strongly enhances the transcriptional activity of the FixJD54N protein both in vivo and in vitro. We present evidence that FixJD54N transcriptional activity is enhanced by phosphorylation of an alternate residue in a reaction that requires FixL and ATP and is not affected by oxygen. We also demonstrate the key role of Asp-54 of FixJ in oxygen signal transduction.
在苜蓿根瘤菌中，固氮基因的转录在双组分调节系统FixLJ的控制下，于缺氧条件下被诱导。FixJ是一种转录激活因子，其活性通过磷酸化而显著增强，而FixL是一种血蛋白激酶，它根据氧气的可利用性来控制磷酸化FixJ的水平。我们发现，一种突变的FixJ蛋白FixJD54N，其中假定的磷酸化位点（天冬氨酸54）被替换为天冬酰胺，其磷酸化受到FixL的强烈影响，并且无法从低分子量磷酸供体乙酰磷酸中检测到其磷酸化。出乎意料的是，FixL在体内和体外均强烈增强了FixJD54N蛋白的转录活性。我们提供的证据表明，在一个需要FixL和ATP且不受氧气影响的反应中，FixJD54N的转录活性通过一个替代残基的磷酸化而增强。我们还证明了FixJ的天冬氨酸54在氧气信号转导中的关键作用。
