External Stresses Affect Gonococcal Type 4 Pilus Dynamics

Mechanism of secretion of TcpF by the Vibrio cholerae toxin-coregulated pilus

Many bacteria possess dynamic filaments called Type IV pili (T4P) that perform diverse functions in colonization and dissemination, including host cell adhesion, DNA uptake, and secretion of protein substrates-exoproteins-from the periplasm to the extracellular space. The Vibrio cholerae toxin-coregulated pilus (TCP) and the enterotoxigenic Escherichia coli CFA/III pilus each mediates export of a single exoprotein, TcpF and CofJ, respectively. Here, we show that the disordered N-terminal segment of mature TcpF is the export signal (ES) recognized by TCP. Deletion of the ES disrupts secretion and causes TcpF to accumulate in the V. cholerae periplasm. The ES alone can mediate export of Neisseria gonorrhoeae FbpA by V. cholerae in a T4P-dependent manner. The ES is specific for its autologous T4P machinery as CofJ bearing the TcpF ES is exported by V. cholerae, whereas TcpF bearing the CofJ ES is not. Specificity is mediated by binding of the ES to TcpB, a minor pilin that primes pilus assembly and forms a trimer at the pilus tip. Finally, the ES is proteolyzed from the mature TcpF protein upon secretion. Together, these results provide a mechanism for delivery of TcpF across the outer membrane and release into the extracellular space.

Direct visualization of sequence-specific DNA binding by gonococcal type IV pili

Neisseria gonorrhoeae, the causative agent of gonorrhoea, is a major burden on global healthcare systems, with an estimated ~80-90 million new global cases annually. This burden is exacerbated by increasing levels of antimicrobial resistance, which has greatly limited viable antimicrobial therapies. Decreasing gonococcal drug susceptibility has been driven largely by accumulation of chromosomal resistance determinants, which can be acquired through natural transformation, whereby DNA in the extracellular milieu is imported into cells and incorporated into the genome by homologous recombination. N. gonorrhoeae possesses a specialized system for DNA uptake, which strongly biases transformation in favour of DNA from closely related bacteria by recognizing a 10-12 bp DNA uptake sequence (DUS) motif, which is highly overrepresented in their chromosomal DNA. This process relies on numerous proteins, including the DUS-specific receptor ComP, which assemble retractile protein filaments termed type IV pili (T4P) extending from the cell surface, and one model for neisserial DNA uptake proposes that these filaments bind DNA in a DUS-dependent manner before retracting to transport DNA into the periplasm. However, conflicting evidence indicates that elongated pilus filaments may not have such a direct role in DNA binding uptake as this model suggests. Here, we quantitatively measured DNA binding to gonococcal T4P fibres by directly visualizing binding complexes with confocal fluorescence microscopy in order to confirm the sequence-specific, comP-dependent DNA binding capacity of elongated T4P fibres. This supports the idea that pilus filaments could be responsible for initially capturing DNA in the first step of sequence-specific DNA uptake.

Mobility of extracellular DNA within gonococcal colonies

Transformation enables bacteria to acquire genetic information from extracellular DNA (eDNA). Close proximity between bacteria in colonies and biofilms may inhibit escape of eDNA from the colony but it also hinders its diffusion between donor and recipient. In this study, we investigate the mobility of DNA within colonies formed by Neisseria gonorrhoeae, and relate it to transformation efficiency. We characterize the penetration dynamics of fluorescent DNA into the colony at a time scale of hours and find that 300 bp fragments diffuse through the colony without hindrance. For DNA length exceeding 3 kbp, a concentration gradient between the edge and the center of the colony develops, indicating hindered diffusion. Accumulation of DNA within the colony increases with increasing DNA length. The presence of the gonococcal DNA uptake sequence (DUS), which mediates specific binding to type 4 pili (T4P) and uptake into the cell, steepens the radial concentration gradient within the colony, suggesting that the DUS reduces DNA mobility. In particular, DNA of N. gonorrhoeae containing multiple DUS is trapped at the periphery. Under conditions, where DUS containing DNA fragments readily enter the colony center, we investigate the efficiency of transformation. We show that despite rapid diffusion of DNA, the transformation is limited to the edge of young colonies. We conclude that DNA mobility depends on DNA length and specific binding mediated by the DUS, resulting in restricted mobility of gonococcal DNA. Yet gonococcal colonies accumulate DNA, and may therefore act as a reservoir for eDNA.

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DNA fragments encompassing the length of a typical operon efficiently penetrate bacterial colonies.

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Bacterial colonies accumulate eDNA with an efficiency that depends on the length and the DNA uptake sequence.

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Genomic DNA from a distinct species spreads efficiently through gonococcal colonies, while gonococcal DNA and DNA from a closely related species are trapped.

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Transformation is most efficient at the periphery of freshly assembled gonococcal colonies.