Molecular characterization of the MgaSpn transcriptional regulator of "Streptcoccus neumoniae"

Resumen

[EN] Bacteria usually live in habitats of changing conditions. During infection, pathogenic bacteria must be able to survive in different environments encountered as the pathogen progresses through its host. This adaptation requires sensing the relevant extracellular signals and linking them to a coordinate change in the expression of genes, which encode factors appropriate to the given situation. Global transcriptional regulators that respond to specific environmental signals are key elements in such regulatory networks. Bacteria often use classical two-component signal transduction systems (TCSs) to link the environmental signals to adaptive responses (Stock et al., 2000). Moreover, in addition to TCSs, stand-alone response regulators have been implicated in the global regulation of virulence gene expression. The term stand-alone has been used to define global transcriptional regulators that (i) are not associated to a membrane-bound sensor histidine kinase, (ii) their activity and/or intracellular concentration changes in response to specific external stimuli and (iii) their signal transduction components have yet to be fully defined (McIver, 2009). The Gram-positive (G+) bacterium Streptococcus pneumoniae, commonly called the pneumococcus, is a member of the normal human nasopharyngeal flora, where it exists asymptomatically as a commensal. However, when the immune system weakens, it can cause serious diseases such as sinusitis, conjunctivitis, otitis media, meningitis and bacteremia (Kadioglu et al., 2008; van der Poll and Opal, 2009). S. pneumoniae remains as a main cause of morbidity and mortality worldwide as a result of its increasing resistance to antibiotics. Recent data estimate that pneumococcal pneumonia kills annually around 1.2 million children younger than five years, more than AIDS, malaria and tuberculosis combined (www.who.int/mediacentre/factsheets/fs331/en/index.html). Understanding the molecular mechanisms that control the expression of pneumococcal virulence genes in response to environmental stimuli will offer new insights into the pathogenesis of this bacterium. Searching for homologies we found that the genome of the pneumococcal R6 strain (Hoskins et al., 2001), which derives from the D39 clinical isolate (serotype 2), encodes a protein (named MgaSpn by us), which is highly conserved in the pneumococcal strains whose genomes have been totally or partially sequenced. At present, MgaSpn is thought to be a member of the Mga/AtxA family of global response regulators, which includes the Mga and the AtxA virulence regulators encoded by the G+ pathogens S. pyogenes (the Group A Streptococcus, GAS) and Bacillus anthracis, respectively. MgaSpn shares 42.6% of similarity and 21.4% of identity with Mga and 39.9% of similarity and 20.7% of identity with AtxA. Regarding the Mga regulator, it controls the expression of approximately 10% of the GAS genome during the exponential growth phase (Ribardo and McIver, 2006). Mga activates directly the transcription of several virulence genes, which encode factors important for adherence and internalization into non-phagocytic cells, as well as factors that enable the bacterium to evade the host immune responses. Mga also activates the expression of its own gene (McIver et al., 1999; McIver, 2009). In vitro studies using a His-tagged Mga showed that it binds to regions located upstream of the target promoters with low sequence identity.(Full summary in attached document)