Na+(Li+)/branched-chain amino acid cotransport in Pseudomonas aeruginosa

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

The branched-chain amino acids (BCAAs [Ile, Leu, and Val]) represent important nutrients in bacterial physiology, with roles that range from supporting protein synthesis to signaling and fine-tuning the adaptation to amino acid starvation. In some pathogenic bacteria, the adaptation to amino acid starvation includes induction of virulence gene expression: thus, BCAAs support not only proliferation during infection, but also the evasion of host defenses.

The branched-chain amino acids (BCAAs [Ile, Leu, and Val]) represent important nutrients in bacterial physiology, with roles that range from supporting protein synthesis to signaling and fine-tuning the adaptation to amino acid starvation. In some pathogenic bacteria, the adaptation to amino acid starvation includes induction of virulence gene expression: thus, BCAAs support not only proliferation during infection, but also the evasion of host defenses. A body of research has accumulated over the years to describe the multifaceted physiological roles of BCAAs and the mechanisms bacteria use to maintain their intracellular levels. More recent studies have focused on understanding how fluctuations in their intracellular levels impact global regulatory pathways that coordinate the adaptation to nutrient limitation, especially in pathogenic bacteria. In this minireview, we discuss how these studies have refined the individual roles of BCAAs, shed light on how BCAA auxotrophy might promote higher sensitivity to exogenous BCAA levels, and revealed pathogen-specific responses to BCAA deprivation. These advancements improve our understanding of how bacteria meet their nutritional requirements for growth while simultaneously remaining responsive to changes in environmental nutrient availability to promote their survival in a range of environments.

Repression of branched-chain amino acid synthesis in Staphylococcus aureus is mediated by isoleucine via CodY, and by a leucine-rich attenuator peptide

Staphylococcus aureus requires branched-chain amino acids (BCAAs; isoleucine, leucine, valine) for protein synthesis, branched-chain fatty acid synthesis, and environmental adaptation by responding to their availability via the global transcriptional regulator CodY. The importance of BCAAs for S. aureus physiology necessitates that it either synthesize them or scavenge them from the environment. Indeed S. aureus uses specialized transporters to scavenge BCAAs, however, its ability to synthesize them has remained conflicted by reports that it is auxotrophic for leucine and valine despite carrying an intact BCAA biosynthetic operon. In revisiting these findings, we have observed that S. aureus can engage in leucine and valine synthesis, but the level of BCAA synthesis is dependent on the BCAA it is deprived of, leading us to hypothesize that each BCAA differentially regulates the biosynthetic operon. Here we show that two mechanisms of transcriptional repression regulate the level of endogenous BCAA biosynthesis in response to specific BCAA availability. We identify a trans-acting mechanism involving isoleucine-dependent repression by the global transcriptional regulator CodY and a cis-acting leucine-responsive attenuator, uncovering how S. aureus regulates endogenous biosynthesis in response to exogenous BCAA availability. Moreover, given that isoleucine can dominate CodY-dependent regulation of BCAA biosynthesis, and that CodY is a global regulator of metabolism and virulence in S. aureus, we extend the importance of isoleucine availability for CodY-dependent regulation of other metabolic and virulence genes. These data resolve the previous conflicting observations regarding BCAA biosynthesis, and reveal the environmental signals that not only induce BCAA biosynthesis, but that could also have broader consequences on S. aureus environmental adaptation and virulence via CodY.

Phospholipids stabilize the secondary structure of the sodium-coupled branched-chain amino acid carrier of Pseudomonas aeruginosa

For functional reconstitution of bacterial cotransporters (carriers or permeases) including the sodium-coupled branched-chain amino acid carrier (LIV-II carrier) of Pseudomonas aeruginosa, the presence of phospholipid is required through the process of solubilization and purification of the transporters from the bacterial membranes, suggesting the possibility that phospholipid may stabilize the structure of the cotransporter proteins to be in a functional form. In this study, this possibility was examined by studying the effect of denaturant on the secondary structure of the LIV-II carrier purified in the absence and presence of phospholipid using circular dichroism (CD) spectroscopy. CD spectra of the purified LIV-II carrier solubilized in n-octyl-beta-D-glucopyranoside (OG), OG/dioleoylphosphatidylethanolamine (DOPE)/dioleoylphosphatidylglycerol (DOPG) mixture, and dispersed into DOPE/DOPG small unilamellar vesicles were measured in the absence of denaturant. The three spectra were very similar and had a trough at 222 nm with mean residue molar ellipticity of -23000 deg.cm(2)/dmol and a shoulder at 208 nm. CD spectral analyses with three different methods (S.W. Provencher, J. Glöckner, Estimation of globular protein secondary structure from circular dichroism, Biochemistry 20 (1981) 33-37; J.Y. Yang, C.-S.C. Wu, H.Z. Martinez, Calculation of protein conformation from circular dichroism, Methods Enzymol. 130 (1986) 208-269; N. Sreerama, R.W. Woody, A self-consistent method for the analysis of protein secondary structure from circular dichroism, Anal. Biochem. 209 (1993) 32-44) revealed that the LIV-II carrier solubilized in OG/DOPE/DOPG mixture contained 69-75% alpha-helix and 0-9% beta-sheet. Addition of 6 M guanidine hydrochloride decreased 48% of the amplitude at 222 nm of the CD spectrum of the carrier solubilized in OG alone and 9-14% of the CD amplitude of the carrier solubilized in OG/DOPE/DOPG or OG/dioleoylphosphatidylcholine mixture and dispersed in liposomes composed of DOPE/DOPG. These results show that the ordered secondary structure of the LIV-II carrier is partially unfolded in OG without phospholipid by denaturant but is greatly stabilized with phospholipids with oleoyl chains independently of their polar head group composition and suggest that the alpha-helical structure of the carrier is mainly embedded in the lipid environment.

Cloning and sequencing of a novel Na+/H+ antiporter gene from Pseudomonas aeruginosa

We cloned a gene for Na+/H+ antiporter from chromosomal DNA of Pseudomonas aeruginosa. Introduction of the gene into host Escherichia coli mutant cells lacking all of the major Na+/H+ antiporters enabled the cells to grow in the presence of 0.2 M NaCl, although the original host cells could not. Membrane vesicles prepared from cells of the transformant possessing the cloned gene showed Na+/H+ antiport activity. As a result of DNA sequencing, we found one open reading frame (nhaP). The deduced amino acid sequence suggests that the Na+/H+ antiporter (NhaP) of P. aeruginosa consists of 424 amino acid residues with molecular mass of 45486 Da, and hydropathy analysis suggested the presence of 12 putative transmembrane domains. We found no bacterial Na+/H+ antiporter which showed significant sequence similarity with the NhaP in the protein sequence database. The NhaP showed partial sequence similarity with animal Na+/H+ exchangers. Thus, the NhaP of P. aeruginosa is unique among bacterial antiporters.

Isolation of the braZ gene encoding the carrier for a novel branched-chain amino acid transport system in Pseudomonas aeruginosa PAO

The braZ gene for a novel branched-chain amino acid transport system in Pseudomonas aeruginosa PAO was isolated and characterized. Determination of the nucleotide sequence showed that the braZ gene comprises 1,311 nucleotides specifying a protein of 437 amino acids. Hydropathy analysis suggested that the product is an integral membrane protein with 12 membrane-spanning segments. The amino acid sequence showed extensive homology to those of the braB and brnQ gene products, branched-chain amino acid carriers of P. aeruginosa and Salmonella typhimurium, respectively. By using the T7 RNA polymerase-promoter system, the braZ gene product was identified as a protein of an apparent Mr of 34,000 on a sodium dodecyl sulfate-polyacrylamide gel. Properties of the transport system encoded by braZ were studied by using P. aeruginosa PAO3537, defective in both the high- and low-affinity branched-chain amino acid transport systems (LIV-I and LIV-II, respectively). The transport system encoded by braZ was found to be another effective branched-chain amino acid transport system in P. aeruginosa PAO and was thus designated as LIV-III. This system is specific for isoleucine and valine, giving the same Km value of 12 microM for these amino acids. The system was found, however, to have a very low affinity for leucine, with a Km value of 150 microM, which contrasts with the substrate specificities of LIV-I and LIV-II.