Merrick M J
ARC Unit of Nitrogen Fixation, University of Sussex, Brighton, UK.
EMBO J. 1983;2(1):39-44. doi: 10.1002/j.1460-2075.1983.tb01377.x.
The ntrC and nifA gene products of Klebsiella pneumoniae are transcriptional activators involved in general nitrogen control and nif-specific regulation, respectively. Multicopy plasmids expressing either ntrC or nifA from a foreign promoter were used to study the relationship between these two genes and ntrA. The nifA product substituted for ntrC product in activation of a number of genes including nifLA, hutUH and genes for arginine and proline utilisation. NtrC could not substitute for nifA in transcriptional activation of the nifHDKY operon. In ntrA- strains, neither the ntrC nor the nifA product functioned to activate transcription of nif promoters. In vitro transcription/translation studies with plasmid clones demonstrated similar levels of expression of ntrC and nifA in ntr+ and ntrA- S-30 extracts. Hence, lack of activator function in an ntrA mutant indicates that both the ntrC and nifA products require a functional ntrA for activity. When expressed from foreign promoters, both the ntrC and nifA products were active in conditions which would normally repress nif expression. Hence, the ntrA product was apparently not limiting in these conditions.
肺炎克雷伯菌的ntrC和nifA基因产物分别是参与一般氮控制和nif特异性调控的转录激活因子。使用从外源启动子表达ntrC或nifA的多拷贝质粒来研究这两个基因与ntrA之间的关系。nifA产物在激活包括nifLA、hutUH以及精氨酸和脯氨酸利用相关基因在内的多个基因时可替代ntrC产物。在nifHDKY操纵子的转录激活中，NtrC不能替代nifA。在ntrA⁻菌株中，ntrC产物和nifA产物均不能激活nif启动子的转录。用质粒克隆进行的体外转录/翻译研究表明，在ntr⁺和ntrA⁻的S-30提取物中ntrC和nifA的表达水平相似。因此，ntrA突变体中激活因子功能的缺失表明ntrC产物和nifA产物都需要功能性的ntrA才能发挥活性。当从外源启动子表达时，ntrC产物和nifA产物在通常会抑制nif表达的条件下均具有活性。因此，在这些条件下ntrA产物显然不是限制因素。
