The iron-responsive sRNA IsrR of Staphylococcus aureus is involved in the regulation of virulence gene expression, in particular via the Sae system

The Gram-positive pathogen Staphylococcus aureus colonizes the nasal mucosa of ~30% of the human population, but also causes various diseases. Adaptation to changing host conditions, such as severe restriction of iron availability, requires precise regulation of genes involved in virulence and metabolic functions.

Therefore, S. aureus possesses various adaptive mechanisms, particularly iron acquisition systems, which are controlled by the global regulator Fur (ferric-uptake regulator) and induced upon iron limitation. In many bacteria, Fur-dependent regulatory RNAs (sRNAs) mediate an iron-sparing response by inhibiting the synthesis of non-essential iron-containing proteins. Recently, the sRNA IsrR (iron sparing response regulator; former S596) was identified [1,2] and its requirement for full S. aureus virulence in a mouse model was shown [2]. We identified the global IsrR targetome by combining proteomic and in silico prediction approaches [3], which, however, does not fully explain the underlying mechanism of this phenotype.

We noticed a positive association of isrR expression with α-hemolysin (Hla) activity of S. aureus HG001. Subsequently, we performed a mass spectrometry-based secretome analysis to investigate the influence of IsrR on virulence factor production. IsrR-expressing and non-expressing strains were compared under iron-limited and -sufficient conditions. To address differences in the amount of secreted proteins depending on the growth phase and the strain background, a normalization strategy based on an external 15N-labeled standard was employed for robust relative quantification. The SaeR regulon was positively influenced by IsrR and thus, we investigated the interplay of IsrR and the major virulence regulator SaeR. IsrR has no direct effect on the SaeRS protein levels but is likely involved in the activation of the Sae system.

Concluding, we demonstrate that IsrR positively influences expression of numerous secreted virulence factors particularly belonging to the SaeR regulon and the heme uptake system (e.g. IsdB). Thereby, IsrR establishes a link between the iron limitation response and virulence in S. aureus.

[1] Mäder et al. (2016) PLoS Genet. 12(4):e1005962. [2] Coronel-Tellez et al. (2022) Acids Res. 50(15):8529-8546. [3] Ganske et al. (2024) Front Microbiol. 15:1439352.