Pneumococcal capsule expression is controlled through a conserved, distal cis-regulatory element during infection

Author summaryStreptococcus pneumoniae (the pneumococcus) is a Gram-positive pathogen that causes significant morbidity and mortality in the US and worldwide. The pneumococcus first colonizes the upper respiratory tract asymptomatically, and then drops into the lung to cause pneumonia, followed by sepsis (blood infection), where patient mortality rates exceed 20%. Studies have indicated that the reason S. pneumoniae can survive in the blood so well is due to its protective polysaccharide capsule. This vital virulence factor acts as a sugary coat which shields the bacterium from immune detection, opsonization, and phagocytosis. However, how this metabolically expensive virulence factor is regulated during colonization, lung infection, and sepsis has remained enigmatic. Here we describe two transcription factors, SpxR and CpsR, which repress capsule biosynthesis in the airways through a small, conserved, regulatory piece of DNA (the 37-CE) until the onset of sepsis-where repression is relieved, allowing for increased capsule biosynthesis, and by association, the high mortality associated with this condition. The 37-CE varies considerably among the 100+ pneumococcal serotypes, each of which has a chemically-distinct capsule composition. This may, in part, explain the variation in serotype-dependent disease severity we have observed for decades. Streptococcus pneumoniae (the pneumococcus) is the major cause of bacterial pneumonia in the US and worldwide. Studies have shown that the differing chemical make-up between serotypes of its most important virulence factor, the capsule, can dictate disease severity. Here we demonstrate that control of capsule synthesis is also critical for infection and facilitated by two broadly conserved transcription factors, SpxR and CpsR, through a distal cis-regulatory element we name the 37-CE. Strikingly, changing only three nucleotides within this sequence is sufficient to render pneumococcus avirulent. Using in vivo and in vitro approaches, we present a model where SpxR interacts as a unique trimeric quaternary structure with the 37-CE to enable capsule repression in the airways. Considering its dramatic effect on infection, variation of the 37-CE between serotypes suggests this molecular switch could be a critical contributing factor to this pathogen's serotype-specific disease outcomes.