Zhang Shijie, Shu Xueqin, Sun Baolin
CAS Key Laboratory of Innate Immunity and Chronic Disease, School of Life Sciences and Medical Center, University of Science and Technology of China, Hefei, Anhui 230027, China.
CAS Key Laboratory of Innate Immunity and Chronic Disease, School of Life Sciences and Medical Center, University of Science and Technology of China, Hefei, Anhui 230027, China; Hefei National Laboratory for Physical Sciences at Microscale, Hefei, Anhui 230027, China.
Int J Med Microbiol. 2016 Sep;306(6):406-14. doi: 10.1016/j.ijmm.2016.05.008. Epub 2016 May 13.
Methicillin-resistant Staphylococcus aureus (MRSA) is a worldwide pathogen that is resistant to practically the entire class of β-lactam antibiotics due to the presence of the mecA gene. The mecA gene is located on a large mobile genetic element referred to as staphylococcal cassette chromosome mec (SCCmec), and the excision and integration of SCCmec are mediated by the Ccr recombinase encoded by ccrAB or ccrC, which are also located on SCCmec. Previous studies have shown that the ccrAB genes are only expressed in a minority of cells and that their expression levels can be affected by certain environmental stimuli, but the molecular mechanisms controlling these phenotypes remain elusive. Here, we found that overexpression of SigB can dramatically enhance ccrA transcription and SCCmec excision in MRSA strain N315, revealing an important role for this alternative sigma factor in the lateral transfer of SCCmec. Further primer extension-blot analysis and 5'RACE (Rapid Amplification of cDNA Ends) indicated that an unrecognized SigB-dependent promoter region, which exists in certain SCCmec type II and IV strains, is responsible for the enhancement, and the ccrAB genes are in fact transcribed in a two-promoter pattern with a low activity of the SigB-dependent promoter under normal growth conditions.
耐甲氧西林金黄色葡萄球菌（MRSA）是一种全球范围内的病原体，由于存在mecA基因，它对几乎所有β-内酰胺类抗生素都具有抗性。mecA基因位于一个称为葡萄球菌盒式染色体mec（SCCmec）的大型移动遗传元件上，SCCmec的切除和整合由同样位于SCCmec上的ccrAB或ccrC编码的Ccr重组酶介导。先前的研究表明，ccrAB基因仅在少数细胞中表达，其表达水平会受到某些环境刺激的影响，但控制这些表型的分子机制仍然难以捉摸。在这里，我们发现SigB的过表达可以显著增强MRSA菌株N315中ccrA的转录和SCCmec的切除，揭示了这种替代sigma因子在SCCmec横向转移中的重要作用。进一步的引物延伸印迹分析和5'RACE（cDNA末端快速扩增）表明，在某些II型和IV型SCCmec菌株中存在一个未被识别的SigB依赖性启动子区域，它导致了这种增强，并且ccrAB基因实际上是以双启动子模式转录的，在正常生长条件下SigB依赖性启动子的活性较低。
