Structural studies of regulatory elements in the RNA transcript for streptolysin S associated gene A (sagA) in group A Streptococcus.

Group A Streptococcus releases a cytotoxic peptide, streptolysin S, that targets sensory neurons and induces an inhibition of the immune response to the invasive disease, necrotizing fasciitis (flesh-eating disease). The sagA gene for streptolysin S also produces a small regulatory RNA (sRNA) known as the pleiotropic effector locus (Pel), which regulates the expression of multiple mRNA transcripts associated with virulence. Potential mRNA interactions have been mapped to the 5' and 3' untranslated regions (UTRs) of sagA, which contain conserved, predicted secondary structures that may be regulatory elements. These results suggest that the 5' UTR of sagA plays a key role in the trans-acting regulatory activity of Pel with other virulence factor mRNA transcripts. Differential scanning fluorimetry and RNase T1 digest assays provided experimental evidence for the predicted secondary structures of these elements. Three dimensional models of the Pel/sagA regulatory elements were built using the FARFAR algorithm in the Rosetta molecular modelling suite. The 3' UTR of sagA contains a complex structural element that combines a branched stem-loop structure with a weak rho-independent terminator, which may be regulated by other factors that dictate expression of the entire sag operon required for the maturation and secretion of streptolysin S. Current studies are focused on experimental determination of the Pel/sagA RNA structure and identification of functional interactions with other virulence factors. The development of therapeutic strategies that target regulatory RNA structural elements may be an effective strategy to mitigate the invasive forms of streptococcal infections.