Transcriptional organization and posttranscriptional regulation of the Bacillus subtilis branched-chain amino acid biosynthesis genes
Transcriptional organization and posttranscriptional regulation of the Bacillus subtilis branched-chain amino acid biosynthesis genes
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DOI：
10.1128/jb.186.8.2240-2252.2004
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发表时间：
2004-04-01
影响因子：
3.2
通讯作者：
Homuth, G
中科院分区：
文献类型：
--
作者：
Mäder, U;Hennig, S;Homuth, G
在枯草芽孢杆菌中，支链氨基酸生物合成途径的基因被组织在三个遗传基因座中：ilvBHC-leuABCD（ilv-leu）操纵子、ilvA和ilvD。这些基因以及编码支链氨基酸氨基转移酶的ybgE最近被证明代表了全球转录调节因子CodY的直接靶点。在本研究中，这些基因的转录组织和转录后调控进行了分析。而ybgE和ilvD是单顺反子转录的，ilvA基因与下游的ypmP基因形成双顺反子操纵子，编码未知功能的蛋白质。ypmP基因还直接在共享ilvA启动子的调控模式的启动子之前。ILV-亮氨酸操纵子揭示了复杂的转录后调控：检测到8.5，5.8和1.2 kb的三种mRNA。其中，8.5 kb全长初级转录物表现出最短的半衰期（1.2分钟）。该转录物的内切核糖核酸裂解产生5.8-kb mRNA，其缺乏操纵子的前两个基因的编码序列，并且预测在其5'端携带茎环结构。这种加工产品的半衰期（3分钟）明显长于全长前体。最稳定的转录本（半衰期，7.6分钟）是1.2 kb的mRNA产生的加工事件和外切核酸降解的大成绩单或部分转录终止。该mRNA仅包含ilvC编码序列，预计在其3'端携带进一步稳定的茎环结构。由其不同的稳定性导致的mRNA的非常不同的稳态量也反映在蛋白质水平上：蛋白质组学研究表明，IlvC蛋白的细胞量是ilv-leu操纵子编码的其他蛋白质的10倍。因此，由mRNA加工产生的差异片段稳定性确保了操纵子的各个基因的表达的微调。
In Bacillus subtilis, the genes of the branched-chain amino acids biosynthetic pathway are organized in three genetic loci: the ilvBHC-leuABCD (ilv-leu) operon, ilvA, and ilvD. These genes, as well as ybgE, encoding a branched-chain amino acid aminotransferase, were recently demonstrated to represent direct targets of the global transcriptional regulator CodY. In the present study, the transcriptional organization and posttranscriptional regulation of these genes were analyzed. Whereas ybgE and ilvD are transcribed monocistronically, the ilvA gene forms a bicistronic operon with the downstream located ypmP gene, encoding a protein of unknown function. The ypmP gene is also directly preceded by a promoter sharing the regulatory pattern of the ilvA promoter. The ilv-leu operon revealed complex posttranscriptional regulation: three mRNA species of 8.5, 5.8, and 1.2 kb were detected. Among them, the 8.5-kb full-length primary transcript exhibits the shortest half-life (1.2 min). Endoribonucleolytic cleavage of this transcript generates the 5.8-kb mRNA, which lacks the coding sequences of the first two genes of the operon and is predicted to carry a stem-loop structure at its 5' end. This processing product has a significantly longer half-life (3 min) than the full-length precursor. The most stable transcript (half-life, 7.6 min) is the 1.2-kb mRNA generated by the processing event and exonu-cleolytic degradation of the large transcripts or partial transcriptional termination. This mRNA, which encompasses exclusively the ilvC coding sequence, is predicted to carry a further stable stem-loop structure at its 3' end. The very different steady-state amounts of mRNA resulting from their different stabilities are also reflected at the protein level: proteome studies revealed that the cellular amount of IlvC protein is 10-fold greater than that of the other proteins encoded by the ilv-leu operon. Therefore, differential segmental stability resulting from mRNA processing ensures the fine-tuning of the expression of the individual genes of the operon.