Protein Transfer through an F Plasmid-Encoded Type IV Secretion System Suppresses the Mating-Induced SOS Response.

Area of Science:

• Microbiology
• Molecular Biology
• Bacterial Genetics

Background:

• Bacterial type IV secretion systems (T4SSs) are crucial for transferring mobile genetic elements (MGEs), including antibiotic resistance and virulence genes.
• Conjugative transfer involves the translocation of single-stranded DNA (ssDNA) intermediates, which can trigger the SOS response in recipient cells.

Purpose of the Study:

• To investigate the translocation of plasmid-encoded proteins by the pED208-encoded T4SS (Tra_pED208).
• To determine the role of translocated proteins, specifically PsiB and SSB, in modulating the SOS response during conjugation.
• To elucidate the mechanism by which protein translocation is activated during DNA transfer.

Main Methods:

• Analysis of protein translocation by the Tra_pED208 T4SS using deletion mutants of the pED208 plasmid.
• Assessment of the SOS response in recipient cells following conjugation.
• Investigating the interaction between the relaxosome, TraD, and protein substrates.

Main Results:

• The Tra_pED208 T4SS translocates several plasmid-encoded proteins, including ParA, ParB1, SSB, ParB2, PsiB, and PsiA, to recipient cells.
• Translocation of proteins requires the pED208 relaxosome's engagement with the TraD substrate receptor.
• Intercellular transfer of PsiB and SSB suppresses the mating-induced SOS response, while transfer of PsiA does not.
• Plasmids lacking psiB or ssb elicit a stronger SOS response upon transfer.

Conclusions:

• Conjugative protein translocation by T4SSs is a mechanism for transferring proteins involved in plasmid maintenance.
• Intercellular transfer of PsiB and SSB represents a novel biological function of T4SSs, mitigating the SOS response and potentially influencing bacterial evolution.
• The docking of the DNA substrate with the TraD receptor is critical for activating protein translocation in F-encoded Tra systems.