Translational Control of the SigR-Directed Oxidative Stress Response in Streptomyces via IF3-Mediated Repression of a Noncanonical GTC Start Codon.
Translational Control of the SigR-Directed Oxidative Stress Response in Streptomyces via IF3-Mediated Repression of a Noncanonical GTC Start Codon.
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DOI：
10.1128/mbio.00815-17
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发表时间：
2017-06-13
期刊：
影响因子：
6.4
通讯作者：
Buttner MJ
中科院分区：
文献类型：
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作者：
Feeney MA;Chandra G;Findlay KC;Paget MSB;Buttner MJ
链霉菌的主要氧化应激反应由sigma因子SigR及其同源抗sigma因子RsrA控制，SigR活性在转录和翻译后水平受到多种机制的严格控制。在这里，我们表明sigR有一个非常不寻常的GTC起始密码子，这导致sigR调控的另一个水平，其中sigR翻译被翻译起始因子3 （IF3）抑制。将GTC改变为规范起始密码子导致SigR相对于RsrA过量产生，导致SigR调控不受调控和组成性表达。同样，引入破坏其抑制SigR翻译能力的IF3*突变具有相同的效果。因此，非规范GTC sigR起始密码子及其被IF3抑制对于氧化应激调节系统的正确和正常功能至关重要。sigR和rsrA共转录并翻译偶联，因此假设sigR和rsrA以化学计量量产生。在这里，我们展示了RsrA可以独立于SigR转录和翻译，提供了RsrA通常在超过SigR的情况下产生的证据，并描述了决定SigR-RsrA化学计量的因素。在所有sigma因子-抗sigma因子调节开关中，这两种蛋白的相对丰度对系统的正常运作至关重要。许多西格玛-反西格玛操纵子共转录和翻译偶联，导致一个普遍的假设，即西格玛和反西格玛因子以固定的1:1比例产生。在sigR-rsrA的情况下，我们表明反西格玛因子产生的过量超过西格玛因子，提供缓冲以防止虚假释放西格玛活性。这种过剩部分是因为sigR具有极其罕见的非规范GTC起始密码子，因此，sigR翻译起始受到IF3的抑制。这一发现突出了非规范起始密码子的潜在意义，其中很少有实验表征。它还强调了使用生物信息学方法预测起始密码子的局限性，这些方法严重依赖于ATG， GTG和TTG是唯一允许的起始密码子的假设。
The major oxidative stress response in Streptomyces is controlled by the sigma factor SigR and its cognate antisigma factor RsrA, and SigR activity is tightly controlled through multiple mechanisms at both the transcriptional and posttranslational levels. Here we show that sigR has a highly unusual GTC start codon and that this leads to another level of SigR regulation, in which SigR translation is repressed by translation initiation factor 3 (IF3). Changing the GTC to a canonical start codon causes SigR to be overproduced relative to RsrA, resulting in unregulated and constitutive expression of the SigR regulon. Similarly, introducing IF3* mutations that impair its ability to repress SigR translation has the same effect. Thus, the noncanonical GTC sigR start codon and its repression by IF3 are critical for the correct and proper functioning of the oxidative stress regulatory system. sigR and rsrA are cotranscribed and translationally coupled, and it had therefore been assumed that SigR and RsrA are produced in stoichiometric amounts. Here we show that RsrA can be transcribed and translated independently of SigR, present evidence that RsrA is normally produced in excess of SigR, and describe the factors that determine SigR-RsrA stoichiometry. In all sigma factor-antisigma factor regulatory switches, the relative abundance of the two proteins is critical to the proper functioning of the system. Many sigma-antisigma operons are cotranscribed and translationally coupled, leading to a generic assumption that the sigma and antisigma factors are produced in a fixed 1:1 ratio. In the case of sigR-rsrA, we show instead that the antisigma factor is produced in excess over the sigma factor, providing a buffer to prevent spurious release of sigma activity. This excess arises in part because sigR has an extremely rare noncanonical GTC start codon, and as a result, SigR translation initiation is repressed by IF3. This finding highlights the potential significance of noncanonical start codons, very few of which have been characterized experimentally. It also emphasizes the limitations of predicting start codons using bioinformatic approaches, which rely heavily on the assumption that ATG, GTG, and TTG are the only permissible start codons.