Dai Z, Sirard J C, Mock M, Koehler T M
Department of Microbiology and Molecular Genetics, Medical School, University of Texas, Houston 77030, USA.
Mol Microbiol. 1995 Jun;16(6):1171-81. doi: 10.1111/j.1365-2958.1995.tb02340.x.
Bacillus anthracis plasmid pXO1 carries the structural genes for the three anthrax toxin proteins, cya (edema factor), lef (lethal factor), and pag (protective antigen). Expression of the toxin genes by B. anthracis is enhanced during growth under elevated levels of CO2. This CO2 effect is observed only in the presence of another pXO1 gene, atxA, which encodes a transactivator of anthrax toxin synthesis. Here we show that transcription of atxA does not appear to differ in cells grown in 5% CO2 compared with cells grown in air. Using a new efficient method for gene replacement in B. anthracis, we constructed an atxA-null mutant in which the atxA-coding sequence on pXO1 is replaced with an omega km-2 cassette. Transcription of all three toxin genes is decreased in the absence of atxA. The pag gene possesses two apparent transcription start sites, P1 and P2; only transcripts with 5' ends mapping to P1 are decreased in the atxA-null mutant. Deletion analysis of the pag promoter region indicates that the 111 bp region upstream of the P1 site is sufficient for atxA-mediated activation of this transcript. The cya and lef genes each have one apparent start site for transcription. Transcripts with 5' ends mapping to these sites are not detected in the atxA-null mutant. The atxA-null mutant is avirulent in mice. Moreover, the antibody response to all three toxin proteins is decreased significantly in atxA-null mutant-infected mice. These data suggest that the atxA gene product also regulates toxin gene expression during infection.
炭疽芽孢杆菌质粒pXO1携带三种炭疽毒素蛋白（cya，水肿因子；lef，致死因子；pag，保护性抗原）的结构基因。炭疽芽孢杆菌在二氧化碳水平升高的条件下生长时，毒素基因的表达会增强。只有在另一个pXO1基因atxA（编码炭疽毒素合成的反式激活因子）存在的情况下，才会观察到这种二氧化碳效应。在此我们表明，与在空气中生长的细胞相比，在5%二氧化碳环境中生长的细胞中atxA的转录似乎没有差异。我们使用一种新的高效炭疽芽孢杆菌基因替换方法，构建了一个atxA缺失突变体，其中pXO1上的atxA编码序列被一个ωkm - 2盒式结构所取代。在没有atxA的情况下，所有三种毒素基因的转录均减少。pag基因有两个明显的转录起始位点，P1和P2；在atxA缺失突变体中，只有5'端定位于P1的转录本减少。对pag启动子区域的缺失分析表明，P1位点上游111 bp的区域足以实现atxA介导的该转录本的激活。cya和lef基因各自有一个明显的转录起始位点。在atxA缺失突变体中未检测到5'端定位于这些位点的转录本。atxA缺失突变体在小鼠中无致病性。此外，在感染atxA缺失突变体的小鼠中，对所有三种毒素蛋白的抗体反应显著降低。这些数据表明，atxA基因产物在感染过程中也调节毒素基因的表达。
