Secretion of a pneumococcal type II secretion system pilus correlates with DNA uptake during transformation

Abstract

Streptococcus pneumoniae is a major human pathogen that successfully adapts to the host environment via an efficient uptake system for free DNA liberated from other organisms in the upper respiratory tract, facilitating immune evasion and drug resistance. Although the initial signaling events leading to pneumococcal competence for DNA transformation and the fate of DNA when it has been taken up have been extensively studied, the actual mechanism by which DNA in the environment may traverse the thick capsular and cell wall layers remains unknown. Here we visualize that induction of competence results in the formation of a native morphologically distinct pilus structure on the bacterial surface. This plaited pilus is encoded by the competence (com)G locus, and, after assembly, it is rapidly released into the surrounding medium. Heterologous pneumococcal pilus expression in Escherichia coli was obtained by replacing the pulE-K putative pilin genes of the Klebsiella oxytoca type II secretion system with the complete comG locus. In the pneumococcus, the coordinated secretion of pili from the cells correlates to DNA transformation. A model for DNA transformation is proposed whereby pilus assembly "drills" a channel across the thick cell wall that becomes transiently open by secretion of the pilus, providing the entry port for exogenous DNA to gain access to DNA receptors associated with the cytoplasmic membrane.

Non-typeable pneumococci circulating in Portugal are of cps type NCC2 and have genomic features typical of encapsulated isolates

“…As expected, all strains had all competence genes, including the newly described transformation pilus [ 14 , 31 , 39 ]. According to the type of competence stimulating peptide (CSP, encoded by comC ) secreted by pneumococcal strains, strains can be divided in pherotypes.…”

Archaeal type IV pili and their involvement in biofilm formation

“…In gram-positive bacteria, the assembly of a competence system that facilitates DNA uptake requires the presence of the PilD homolog, ComC, to process the subunits of a high molecular weight DNA-binding surface structure, as well as the PilB and PilC homologs, ComGA and ComGB, respectively. DNA binding pili have been visualized in Streptococcus pneumoniae ( Laurenceau et al, 2013 ; Balaban et al, 2014 ). However, in the well-studied Com system of Bacillus subtilis these structures do not promote adhesion nor do they form surface filaments under the range of conditions that have been tested ( Chen and Dubnau, 2004 ).…”

Uptake of extracellular DNA: Competence induced pili in natural transformation of Streptococcus pneumoniae

“…These structures are 8–10 nm in diameter and on average ∽100 nm in length. They further demonstrated that E. coli encoding the T2S system structural elements of K. oxytoca can produce similar plaited structures when pulE-K was exchanged to respective homologs of the comG operon present in S. pneumoniae 9 .…”

“…It was further found that pneumococcal ComGC is present in the medium after competence induction that was not a result of bacterial lysis 9 . Furthermore, no active shearing forces were applied in the experimental procedure, suggesting that ComGC is released from the bacteria.…”