Abstract
Membrane surface localized endonuclease EndA of the pulmonary pathogen Streptococcus pneumoniae (pneumococcus) is required for both genetic transformation and virulence. Pneumococcus expresses EndA during growth. However, it has been reported that EndA has no access to external DNA when pneumococcal cells are not competent for genetic transformation, and thus, unable to degrade extracellular DNA. Here, by using both biochemical and genetic methods, we demonstrate the existence of EndA-mediated nucleolytic activity independent of the competence state of pneumococcal cells. Pneumococcal mutants that are genetically deficient in competence development and genetic transformation have extracellular nuclease activity comparable to their parental wild type, including their ability to degrade neutrophil extracellular traps (NETs). The autolysis deficient ΔlytA mutant and its isogenic choline-treated parental wild-type strain D39 degrade extracellular DNA readily, suggesting that partial cell autolysis is not required for DNA degradation. We show that EndA molecules are secreted into the culture medium during the growth of pneumococcal cells, and contribute substantially to competence-independent nucleolytic activity. The competence-independent activity of EndA is responsible for the rapid degradation of DNA and NETs, and is required for the full virulence of Streptococcus pneumoniae during lung infection.
Highlights
EndA is reported to be a membrane-localized pneumococcal endonuclease [1]
To determine if the extracellular DNA degradation by EndA was dependent on the development of competence, we examined the induction of competence by using the pneumococcal strain JC0923 (Table 1) in the presence or absence of competence stimulating peptide 1 (CSP1)
Nucleolytic activity of EndA was only detectable in pneumococcal cells competent for genetic transformation [7,8]
Summary
EndA is reported to be a membrane-localized pneumococcal endonuclease [1]. It was first implicated to play a role in genetic transformation by Kohoutova [2], and subsequently confirmed by Lacks and colleagues [3,4,5]. Acid soluble DNA fragments or nucleotides generated during DNA degradation are released into the culture medium [4], and are only detectable during competence development [7]. These observations were further confirmed by Berge and colleagues [8]. The comEA and cglABCDEFG operons, which encode the apparatus for DNA binding and uptake, are required for EndA-mediated DNA degradation These authors proposed that DNA attaches to the apparatus in competent pneumococcal cells, allowing the membrane-localized EndA to gain access to the extracellular DNA. For pneumococcal cells not under the competent state, EndA is unable to gain access to donor DNA, and is incapable of degrading extracellular DNA
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