Expression, crystallization, and preliminary X-ray crystallographic analysis of putative SpoVG from Staphylococcus aureus

Glutamate-dependent arginine biosynthesis requires the inactivation of spoVG, sarA, and ahrC in Staphylococcus aureus

Genome sequencing has demonstrated that Staphylococcus aureus encodes arginine biosynthetic genes argDCJBFGH synthesizing proteins that mediate arginine biosynthesis using glutamate as a substrate. Paradoxically, however, S. aureus does not grow in a defined, glutamate-replete medium lacking arginine and glucose (CDM-R). Studies from our laboratory have found that specific mutations are selected by S. aureus that facilitate growth in CDM-R. However, these selected mutants synthesize arginine utilizing proline as a substrate rather than glutamate. In this study, we demonstrate that the ectopic expression of the argDCJB operon supports the growth of S. aureus in CDM-R, thus documenting the functionality of this pathway. Furthermore, suppressor mutants of S. aureus JE2 putA::Tn, which is defective in synthesizing arginine from proline, were selected on CDM-R agar. Genome sequencing revealed that these mutants had compensatory mutations within both spoVG, encoding an ortholog of the Bacillus subtilis stage V sporulation protein, and sarA, encoding the staphylococcal accessory regulator. Transcriptional studies document that argD expression is significantly increased when JE2 spoVG sarA was grown in CDM-R. Lastly, we found that a mutation in ahrC was required to induce argD expression in JE2 spoVG sarA when grown in an arginine-replete medium (CDM), suggesting that AhrC also functions to repress argDCJB in an arginine-dependent manner. In conclusion, these data indicate that the argDCJB operon is functional when transcribed in vitro and that SNPs within potential putative regulatory proteins are required to alleviate the repression.IMPORTANCEAlthough Staphylococcus aureus has the capability to synthesize all 20 amino acids, it is phenotypically auxotrophic for several amino acids including arginine. This work identifies putative regulatory proteins, including SpoVG, SarA, and AhrC, that function to inhibit the arginine biosynthetic pathways using glutamate as a substrate. Understanding the ultimate mechanisms of why S. aureus is selected to repress arginine biosynthetic pathways even in the absence of arginine will add to the growing body of work assessing the interactions between metabolism and S. aureus pathogenesis.

Involvement of SpoVG in hemolysis caused by Bacillus subtilis

Bacillus subtilis is a facultative anaerobic Gram-positive non-pathogenic bacterium that includes members displaying hemolytic activity. To identify the genes responsible for hemolysis, a random mariner-based transposon insertion mutant library of B. subtilis 168 was constructed. More than 20,000 colonies were screened for the hypohemolytic phenotype on blood agar plates. One mutant showed significantly less pronounced hemolytic phenotype than the wild type. DNA sequencing and Southern blot analysis showed this mutant has a single transposable element inserted into the open reading frame (ORF) of the spoVG gene; complementation of the spoVG-disrupted mutant with a wild-type copy restored its hemolytic phenotype. It was therefore concluded that the spoVG gene, which plays a role in regulating asymmetric septation during sporulation in B. subtilis, is involved in hemolysis by B. subtilis.

Eubacterial SpoVG homologs constitute a new family of site-specific DNA-binding proteins

A site-specific DNA-binding protein was purified from Borrelia burgdorferi cytoplasmic extracts, and determined to be a member of the highly conserved SpoVG family. This is the first time a function has been attributed to any of these ubiquitous bacterial proteins. Further investigations into SpoVG orthologues indicated that the Staphylococcus aureus protein also binds DNA, but interacts preferentially with a distinct nucleic acid sequence. Site-directed mutagenesis and domain swapping between the S. aureus and B. burgdorferi proteins identified that a 6-residue stretch of the SpoVG α-helix contributes to DNA sequence specificity. Two additional, highly conserved amino acid residues on an adjacent β-sheet are essential for DNA-binding, apparently by contacts with the DNA phosphate backbone. Results of these studies thus identified a novel family of bacterial DNA-binding proteins, developed a model of SpoVG-DNA interactions, and provide direction for future functional studies on these wide-spread proteins.

The σB-dependent yabJ-spoVG operon is involved in the regulation of extracellular nuclease, lipase, and protease expression in Staphylococcus aureus

The alternative sigma factor σ(B) of Staphylococcus aureus is involved in the coordination of the general stress response, expression of virulence determinants, and modulation of antibiotic resistance levels. It controls a large regulon, either directly by recognizing conserved σ(B) promoter sequences or indirectly via σ(B)-dependent elements. The σ(B)-controlled yabJ-spoVG operon encodes two such putative downstream elements. We report here transcriptome analysis in S. aureus Newman, showing that inactivation of the yabJ-spoVG operon had primarily a repressing effect on a small subregulon encoding mainly virulence factors, including a nuclease (nuc), a protease (splE) and a lipase (lip). As a consequence, extracellular nuclease, protease, and lipase activities were reduced in a ΔyabJ-spoVG mutant. trans-complementation by SpoVG was sufficient to restore their reduced phenotypic expression and lowered transcription due to the yabJ-spoVG deletion. It did not restore, however, the changes triggered by σ(B) inactivation, indicating that both regulons only partially overlap, despite the σ(B) dependency of the yabJ-spoVG expression. Thus, σ(B) is likely to control additional, SpoVG-independent factors affecting the expression of numerous hydrolytic enzymes. SpoVG, on the other hand, seems to fine-tune the σ(B)-dependent regulation of a subset of virulence factors by antagonizing the σ(B) effect.