Response from Balaban

I would like to expand on one point raised by Jean Lee: how inhibiting the autoinducer of virulence, RNAIII-activating protein (RAP), can act to suppress staphylococcal infections. The expression of many virulence factors by Staphylococcus aureus is controlled by RNAIII, a regulatory RNA molecule that is encoded by the global regulatory locus agr. RNAIII can be activated by two different autoinducers: by RAP, which is agr independent[1xBalaban, N. and Novick, R.P. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1619–1623CrossRef | PubMed | Scopus (119)See all References, 2xBalaban, N. et al. Science. 1998; 280: 438–440CrossRef | PubMed | Scopus (163)See all References], and by an octapeptide, which is encoded by the agr locus itself[3xGuangyong, J., Beavis, R., and Novick, R.P. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 12055–12059CrossRef | PubMed | Scopus (383)See all References][3]. Because agr is active only from the mid-exponential phase of growth, whereas RAP is continuously produced and secreted, we hypothesize that there may be a two-step rnaiii activation process (Fig. 1Fig. 1). During cell growth, the concentration of RAP (signal A) builds to a threshold level[1xBalaban, N. and Novick, R.P. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1619–1623CrossRef | PubMed | Scopus (119)See all References][1]. It then binds to its receptor, causing a cascade of events that leads to the phosphorylation of a protein called `target of RAP' (TRAP) (N. Balaban et al., unpublished). These events lead to upregulation of sarA, which then upregulates agr (Ref. [4xChien, Y. and Cheung, A.L. J. Biol. Chem. 1998; 273: 2645–2652CrossRef | PubMed | Scopus (113)See all References][4]). Once agr is activated, agrD is expressed, and an octapeptide encoded by this gene (signal B) is produced and secreted[3xGuangyong, J., Beavis, R., and Novick, R.P. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 12055–12059CrossRef | PubMed | Scopus (383)See all References][3]. This peptide then binds to its receptor, AgrC (Ref. [5xLina, G. et al. Mol. Microbiol. 1998; 28: 655–662CrossRef | PubMed | Scopus (144)See all References][5]), resulting in its phosphorylation, in downregulation of TRAP phosphorylation (N. Balaban et al., unpublished) and in upregulation of P3 to transcribe RNAIII. RNAIII induces the genes that encode various virulence factors, leading to production of toxins. RNA-inhibiting protein (RIP) has been proposed to act as an antagonist[1xBalaban, N. and Novick, R.P. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1619–1623CrossRef | PubMed | Scopus (119)See all References][1], blocking RAP from binding to its receptor. Therefore, in the presence of RIP or anti-RAP antibodies, TRAP is not phosphorylated (N. Balaban et al., unpublished), the signal transduction of virulence is not activated and toxins regulated by RNAIII are not produced.Fig. 1Proposed regulatory pathway of autoinduction of virulence in Staphylococcus aureus. Abbreviations: RAP, RNAIII-activating protein; TRAP, target of RAP; TSST-1, toxic shock syndrome toxin.View Large Image | Download PowerPoint SlideIndependent of agr, sar has also been shown to be the primary regulator element that controls the expression of cna, the gene that encodes collagen adhesin[6xGillaspy, A.F. et al. Infect. Immun. 1998; 66: 3170–3178PubMedSee all References][6]. Collagen adhesin plays a major role in the pathogenesis of septic arthritis. Mutations within agr only slightly reduce the incidence and severity of arthritis. Our data indicate that, in the presence of RIP, S. aureus is inhibited from causing either septic arthritis or arthritis in mice (A. Tarkowski, V. Collins and N. Balaban, unpublished). Our results suggest that RIP may not only downregulate RNAIII to inhibit the production of toxins but may also regulate the production of collagen adhesin, perhaps by regulating sar.