Patton Toni G, Rice Kelly C, Foster Mary K, Bayles Kenneth W
Department of Microbiology, Molecular Biology and Biochemistry, University of Idaho, Moscow, ID 83844-3052, USA.
Mol Microbiol. 2005 Jun;56(6):1664-74. doi: 10.1111/j.1365-2958.2005.04653.x.
The Staphylococcus aureus cid and lrg operons have previously been shown to affect murein hydrolase activity and antibiotic tolerance. Based on their similarities to the holin family of proteins it was proposed that the functions of the cidA and lrgA gene products are analogous to bacteriophage-encoded holin and antiholin proteins respectively. The cid operon expresses two overlapping transcripts, one that spans the cidA, cidB and cidC genes and whose expression is induced by the acetic acid generated by aerobic growth in the presence of excess glucose, and the other that spans the cidB and cidC genes only and is expressed in a sigma B-dependent manner. In the study presented here, we have focused primarily on the third gene of this operon, cidC. A sequence analysis of the cidC gene product suggested that it encodes a pyruvate oxidase that catalyses the oxidative decarboxylation of pyruvate yielding acetate and CO(2). Indeed, a ferricyanide-based spectrophotometric assay revealed that the cidC mutant produced decreased pyruvate oxidase activity relative to the parental and complemented strains. In the presence of excess glucose the cidC mutant accumulated normal levels of acetic acid in the growth medium, likely because of the activity of the pyruvate dehydrogenase complex. However, in contrast to the wild type and complemented strains, the pH of the cidC mutant culture began to increase gradually until it was able to utilize the acetate for a secondary round of growth. Finally, a mutation in cidA caused reduced cell lysis in stationary phase but only minimally affected cell death. These results indicate that the cidC gene product is involved in the generation of acetic acid that contributes to the cell death and lysis that occurs in high-glucose stationary phase cultures, while the cidA gene product, a putative holin, controls lysis of the dying cells.
先前的研究表明，金黄色葡萄球菌的cid和lrg操纵子会影响胞壁质水解酶活性和抗生素耐受性。基于它们与孔蛋白家族蛋白质的相似性，有人提出cidA和lrgA基因产物的功能分别类似于噬菌体编码的孔蛋白和抗孔蛋白。cid操纵子表达两种重叠转录本，一种跨越cidA、cidB和cidC基因，其表达由在过量葡萄糖存在下需氧生长产生的乙酸诱导，另一种仅跨越cidB和cidC基因，并以σB依赖性方式表达。在本研究中，我们主要关注该操纵子的第三个基因cidC。对cidC基因产物的序列分析表明，它编码一种丙酮酸氧化酶，催化丙酮酸的氧化脱羧反应生成乙酸和CO₂。事实上，基于铁氰化物的分光光度法检测显示，相对于亲本菌株和互补菌株，cidC突变体产生的丙酮酸氧化酶活性降低。在过量葡萄糖存在的情况下，cidC突变体在生长培养基中积累的乙酸水平正常，这可能是由于丙酮酸脱氢酶复合物的活性。然而，与野生型和互补菌株不同，cidC突变体培养物的pH值开始逐渐升高，直到它能够利用乙酸进行第二轮生长。最后，cidA中的突变导致稳定期细胞裂解减少，但对细胞死亡的影响最小。这些结果表明，cidC基因产物参与乙酸的生成，乙酸导致高葡萄糖稳定期培养物中发生的细胞死亡和裂解，而cidA基因产物（一种假定的孔蛋白）控制垂死细胞的裂解。
