Role of BrnQ1 and BrnQ2 in Branched-Chain Amino Acid Transport and Virulence in Staphylococcus aureus

Abstract

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. Staphylococcus aureus is a highly successful human pathogen that succeeds at infecting virtually every body site, causing skin, soft tissue, respiratory, bone, joint, and endovascular infections (1). Maintenance of metabolic homeostasis is important for its infection process, as the majority of genes necessary for infection identified in large-scale signature-tagged mutagenesis (STM) screens fall into the categories of metabolism, transport, and biosynthesis (2, 3). Acquisition of host-derived nutrients, specifically amino acids, appears to be an important mechanism of meeting nutritional needs, as a large proportion of attenuated STM strains contain mutations in amino acid transporters (3).Amino acid transporters are ubiquitous in bacteria and are typically selective for transporting either one amino acid or several amino acids with related structures (4). The branched-chain amino acids (BCAAs; Ile, Leu, and Val) are hydrophobic amino acids typically found in the core in globular proteins or in the transmembrane domain in cell surface proteins (5). The transport mechanisms described for acquisition of the BCAAs in bacteria include secondary transporters belonging to the Leu, Ile, Val:cation symporter (LIVCS) family (e.g., BrnQ, BraB, BraZ, and BcaP) (4, 6-9) and the LIV-I ABC transporter (10-13). With the exception of an ABC transporter in Streptococcus pneumoniae (14), secondary transport, which couples the movement of an ion...

Transcriptomic Analysis of Laribacter hongkongensis Reveals Adaptive Response Coupled with Temperature

“…Branched-chain amino acid (BCAA) leucine, isoleucine and valine are essential low-melting-point fatty acid for bacterial survival, in particular gram-positive bacteria [60]. Interestingly, we noticed that the gene expression of BCAA biosynthesis were similar at both temperatures but those related to BCAA degradation and transportation system were elevated significantly at 20°C (Fig 7).…”

A spectrum of CodY activities drives metabolic reorganization and virulence gene expression in Staphylococcus aureus

“…C). Although these uptake systems have not been biochemically characterized, we speculate that they may differ in affinity as is the case for ILV uptake proteins (Kaiser et al ., ), and our data potentially reveal a strategy in which a low‐affinity system may be rapidly turned on to import relatively abundant methionine. As amino acids including methionine become increasingly scarce, a high‐affinity uptake system may be induced along with the genes for methionine synthesis.…”

Branching Out: Alterations in Bacterial Physiology and Virulence Due to Branched-Chain Amino Acid Deprivation

“…S. aureus encodes two additional brnQ homologues: BrnQ2, an Ile-dedicated transporter, and BrnQ3, which has no observed BCAA transport function (107, 119). A LIV-I system with a substrate binding protein able to bind BCAAs has been described in S.…”

“…S. aureus requires both the BrnQ1 and BcaP transporters for optimal fitness during systemic infection and nasal colonization (107, 119). BCAA acquisition also likely contributes to S.…”