Kaiser Julienne C, Omer Sameha, Sheldon Jessica R, Welch Ian, Heinrichs David E
Department of Microbiology and Immunology, University of Western Ontario, London, Ontario, Canada.
Department of Animal Care and Veterinary Services, University of Western Ontario, London, Ontario, Canada.
Infect Immun. 2015 Mar;83(3):1019-29. doi: 10.1128/IAI.02542-14. Epub 2014 Dec 29.
The branched-chain amino acids (BCAAs; Ile, Leu, and Val) not only are important nutrients for the growth of Staphylococcus aureus but also are corepressors for CodY, which regulates virulence gene expression, implicating BCAAs as an important link between the metabolic state of the cell and virulence. BCAAs are either synthesized intracellularly or acquired from the environment. S. aureus encodes three putative BCAA transporters, designated BrnQ1, BrnQ2, and BrnQ3; their functions have not yet been formally tested. In this study, we mutated all three brnQ paralogs so as to characterize their substrate specificities and their roles in growth in vitro and in vivo. We demonstrated that in the community-associated, methicillin-resistant S. aureus (CA-MRSA) strain USA300, BrnQ1 is involved in uptake of all three BCAAs, BrnQ2 transports Ile, and BrnQ3 does not have a significant role in BCAA transport under the conditions tested. Of the three, only BrnQ1 is essential for USA300 to grow in a chemically defined medium that is limited for Leu or Val. Interestingly, we observed that a brnQ2 mutant grew better than USA300 in media limited for Leu and Val, owing to the fact that this mutation leads to overexpression of brnQ1. In a murine infection model, the brnQ1 mutant was attenuated, but in contrast, brnQ2 mutants had significantly increased virulence compared to that of USA300, a phenotype we suggest is at least partially linked to enhanced in vivo scavenging of Leu and Val through BrnQ1. These data uncover a hitherto-undiscovered connection between nutrient acquisition and virulence in CA-MRSA.
支链氨基酸（BCAAs；异亮氨酸、亮氨酸和缬氨酸）不仅是金黄色葡萄球菌生长的重要营养素，也是CodY的共抑制因子，CodY可调节毒力基因表达，这表明BCAAs是细胞代谢状态与毒力之间的重要联系。BCAAs可在细胞内合成或从环境中获取。金黄色葡萄球菌编码三种假定的BCAA转运蛋白，分别命名为BrnQ1、BrnQ2和BrnQ3；它们的功能尚未经过正式测试。在本研究中，我们对所有三个brnQ旁系同源基因进行了突变，以表征它们的底物特异性以及在体外和体内生长中的作用。我们证明，在社区获得性耐甲氧西林金黄色葡萄球菌（CA-MRSA）菌株USA300中，BrnQ1参与所有三种BCAAs的摄取，BrnQ2转运异亮氨酸，在测试条件下，BrnQ3在BCAA转运中没有显著作用。在这三种转运蛋白中，只有BrnQ1对USA300在亮氨酸或缬氨酸受限的化学限定培养基中生长至关重要。有趣的是，我们观察到，在亮氨酸和缬氨酸受限的培养基中，brnQ2突变体比USA300生长得更好，因为这种突变导致brnQ1过表达。在小鼠感染模型中，brnQ1突变体的毒力减弱，但相比之下，brnQ2突变体的毒力比USA300显著增强，我们认为这种表型至少部分与通过BrnQ1增强体内亮氨酸和缬氨酸的清除有关。这些数据揭示了CA-MRSA中营养获取与毒力之间迄今未被发现的联系。
