Pseudomonas aeruginosa is a versatile opportunistic bacterial pathogen. Its ability
to successfully colonize various environments and hosts is partly due to the capability
of P. aeruginosa to outcompete other bacteria. The Type Six Secretion System (T6SS)
is one key weapon giving a competitive advantage. It is a supramolecular contractile
machine capable of delivering a plethora of potent antibacterial toxins. P. aeruginosa
possesses three T6SSs (H1- to H3-T6SS) and several T6SS-related islands scattered
throughout the genome. Some of these islands encode the T6SS puncturing device,
VgrG, and additional T6SS-related function such as Toxin-Immunity pairs.
The vgrG1b operon is present downstream of the H1-T6SS cluster and is
composed of seven genes. This operon is highly conserved in P. aeruginosa genomes
except for the toxin-immunity gene pair, tse7-tsi7. This study demonstrates the Cterminal DNase activity of Tse7. Moreover, the study shows that delivery of Tse7
requires the H1-T6SS and the N-terminus of Tse7 PAAR domain tops the VgrG1b
spike. Tsi7, protects producing cells from the DNase activity through direct interactions
with Tse7.
Additional putative nucleases were identified (TpnA and TpnB). These nucleases
are encoded in genetic clusters also encoding PAAR proteins. The tpnB cluster
encodes a novel T6SS-chaperone type, TapN, which harbours a DUF4123 domain.
TapN is required for the association of the toxin TpnB to the PAAR (PA3904). Another
protein, PA3906, encoded within the tpnB cluster is also proposed to have a putative
chaperone function by interacting with the PAAR tip.
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Finally, this study investigates the post-translational regulation of the H1-T6SS activity
via the repressor TagF. Two mechanisms of TagF action have been proposed. Firstly,
TagF directly counteracts Fha, a T6SS activator. Secondly, TagF directly interacts with
T6SS components making the so-called baseplate subcomplex, suggesting a new
mode of inhibition by blocking T6SS conformational changes and subsequent sheath
contraction.