Schilke B A, Donohue T J
Department of Bacteriology, University of Wisconsin-Madison 53706.
J Mol Biol. 1992 Jul 5;226(1):101-15. doi: 10.1016/0022-2836(92)90127-6.
In this paper, the response of the transcriptional control region of the Rhodobacter sphaeroides cytochrome c2 gene, cycA, to intermediates in heme biosynthesis was studied. To determine if cycA transcription was regulated by heme availability, several precursors or analogs of tetrapyrroles were tested. Addition of delta-aminolevulinate (ALA), the first committed intermediate in heme biosynthesis, was shown to inhibit cycA transcription initiation at both the upstream and downstream promoter regions. In addition, an ALA auxotroph, which can grow in the presence of high levels of ALA, showed a 5 to 7-fold reduction in steady-state transcription from cycA::lacZYA operon fusions. To identify genetic elements responsible for negative regulation by ALA, trans-acting mutants with increased expression of cycA were isolated that were resistant to growth inhibition by the heme analog cohemin. These cohemin-resistant mutants (Chr) have elevated levels of several cycA transcripts and they contain cycA transcripts that had not previously been detected in wild-type cells. In addition, cycA transcription in the Chr mutants continues after the addition of ALA. Finally, we found that Chr mutants have increased ALA synthase activity, suggesting that synthesis of cytochrome c2 and ALA synthase are controlled by a common gene product whose activity has been modified in these mutants. A model is presented to explain how changes in tetrapyrrole intermediates could provide an effective signal to control both cycA transcription and ALA synthase synthesis in R. sphaeroides.
本文研究了球形红细菌细胞色素c2基因（cycA）转录控制区对血红素生物合成中间产物的反应。为了确定cycA转录是否受血红素可用性的调节，测试了几种四吡咯前体或类似物。结果表明，添加δ-氨基乙酰丙酸（ALA），即血红素生物合成中的第一个关键中间产物，可抑制cycA在上游和下游启动子区域的转录起始。此外，一种ALA营养缺陷型菌株，在高水平ALA存在下能够生长，其cycA::lacZYA操纵子融合体的稳态转录水平降低了5至7倍。为了鉴定负责ALA负调控的遗传元件，分离出了cycA表达增加的反式作用突变体，这些突变体对血红素类似物辅酶血红素的生长抑制具有抗性。这些辅酶血红素抗性突变体（Chr）中几种cycA转录本的水平升高，并且它们含有野生型细胞中以前未检测到的cycA转录本。此外，在添加ALA后，Chr突变体中的cycA转录仍在继续。最后，我们发现Chr突变体的ALA合酶活性增加，这表明细胞色素c2和ALA合酶的合成受一种共同基因产物的控制，该基因产物的活性在这些突变体中发生了改变。本文提出了一个模型，以解释四吡咯中间产物的变化如何能提供一个有效信号，来控制球形红细菌中cycA转录和ALA合酶的合成。
