240 p.-7 tab.-44 fig.[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)[ES] Regulación global de la expresión de genes de virulencia en bacterias patógenas
La patogenicidad de determinadas bacterias puede entenderse como una respuesta de adaptación rápida a los cambios producidos en las condiciones del entorno que las
rodea. La habilidad para detectar y responder a esos cambios es lo que les conferirá cierta ventaja a la hora de colonizar nuevos nichos así como evadir el sistema inmune del organismo infectado. En el caso concreto de las bacterias patógenas, estas respuestas van unidas a cambios en la expresión de determinados genes que codifican factores implicados en virulencia. Normalmente, las bacterias utilizan los sistemas de transducción de señales de dos componentes (TCSs) para conectar los estímulos ambientales con una respuesta adaptativa concreta. Estos sistemas, que están implicados en diversos procesos celulares, se componen de dos proteínas, una histidina quinasa (HK) y un regulador de respuesta (RR). La HK es generalmente una proteína integral de membrana, encargada de captar y responder a un determinado estímulo modificando el estado de fosforilación del RR citosólico. Esta fosforilación provocará cambios conformacionales en el regulador el cual, ahora, podrá actuar como un factor transcripcional activando o reprimiendo la expresión de determinados genes (para una revisión ver Perry et al., 2011). Además de estos sistemas, varios reguladores de respuesta denominados stand-alone han sido implicados en la regulación global de la expresión de genes de virulencia. En general, el término stand-alone hace referencia a i) no están asociados a una HK unida a membrana, ii) su actividad y/o concentración intracelular varía en respuesta a estímulos externos y iii) los mecanismos implicados en la transducción de dichos estímulos no han sido totalmente definidos (McIver, 2009).
Dentro de este grupo de reguladores se encuentra la proteína MgaSpn de
Streptococcus pneumoniae, cuya caracterización molecular ha sido el objetivo principal de este trabajo. En este momento, y como consecuencia de nuestra investigación, se considera que MgaSpn es un miembro de la familia Mga/AtxA de reguladores de respuesta global, que incluye a los reguladores de virulencia Mga (S. pyogenes) y AtxA (Bacillus anthracis).(Ver resumen completo en documento adjunto)The work was supported by grants from the Spanish
Ministry of Education and Science (BFU2006-08487 and fellowship FPI BES-2007-17086), the Community of Madrid/Spanish National Research Council (CCG08 CSIC/SAL-3694) and the Spanish Ministry of Science and Innovation (CSD2008-00013-INTERMODS, BFU2009-11868).Novel plasmid-based genetic tools for the study of promoters and terminators in Streptococcus pneumoniae and Enterococcus faecalis.(https://digital.csic.es/handle/10261/42052)
Activator role of the pneumococcal Mga-Like virulence transcriptional regulator (https://digital.csic.es/handle/10261/60421)
The pneumococcal MgaSpn virulence transcriptional regulator generates multimeric complexes on linear double-stranded DNA
(https://digital.csic.es/handle/10261/95584)Peer reviewe
