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在肠道细菌中,碳代谢总体调控蛋白CRP（也称为cAMP受体蛋白）通过PTS系统介导葡萄糖效应,调控σ70依赖型碳源分解代谢操纵子的转录活性。当自生（联合）固氮的肺炎克氏杆菌（Klebsiellapneumoniae）以不同的碳水化合物作为惟一碳源生长时,其固氮活性明显不同,且这种不同碳源对固氮活性的影响发生在基因表达调控水平上,在近缘的大肠杆菌遗传背景下的进一步研究表明,在cAMP存在时,CRP对肺炎克氏杆菌所有的σ54依赖型nif启动子（nifB,nifE,nifF,nifH,nifJ,nifLA和nifU）都有抑制作用,序列分析结果表明,CRP的抑制作用与其在这些启动子上游的DNA顺式作用元件无直接相关性,对肺炎克氏杆菌crp基因的分离、测序结果显示,其编码的CRP蛋白与已知的大肠杆菌CRP蛋白在序列与功能上高度保守,因此认为不同碳源代谢对固氮基因的调控作用可能是由CRP-cAMP复合体介导的。这一研究工作将碳代谢调控系统和固氮调控系统通过CRP-cAMP耦联在一起,具有重要的生理学意义。
In gut bacteria, the general carbon metabolism regulatory protein CRP (also known as cAMP receptor protein) regulates the transcriptional activity of the o70 operon with a sigma-dependent carbon source by mediating glucose effects through the PTS system. When the self-generating (combined) nitrogen fixation of Klebsiella pneumoniae grows with different carbohydrates as the sole carbon source, its nitrogen fixation activity is significantly different, and the effect of different carbon sources on the nitrogen fixation activity occurs at the level of gene expression , Further studies in the close genetic background of E. coli indicate that all of the sigma-54-dependent nif promoters (nifB, nifE, nifF, nifH, nifJ, nifLA and nifU) of CRP against K. pneumoniae are present in the presence of cAMP The results of sequence analysis showed that there was no direct correlation between the inhibitory effect of CRP and the DNA cis-acting elements upstream of these promoters. The isolation of Klebsiella pneumoniae crp gene showed that the CRP protein encoded by Known Escherichia coli CRP protein highly conserved in sequence and function, so that different carbon metabolism of the nitrogen-fixing gene regulation may be mediated by the CRP-cAMP complex. This work coupled the carbon metabolism control system and nitrogen fixation control system through the CRP-cAMP coupled, has important physiological significance.