Tracking F plasmid TraI relaxase processing reactions provides insight into F plasmid transfer

Area of Science:

• Molecular Biology
• Microbiology
• Genetics

Background:

• F plasmid conjugative transfer is a crucial mechanism for bacterial genetic exchange.
• The F relaxase (TraI) enzyme plays a central role in initiating and resolving DNA cleavage during transfer.
• Understanding the precise mechanism of TraI action is key to elucidating plasmid conjugation.

Purpose of the Study:

• To investigate the role of TraI in both DNA cleavage and re-circularization during F plasmid transfer.
• To determine if TraI is transferred to the recipient cell during conjugation.
• To identify the specific active site residues of TraI involved in these processes.

Main Methods:

• Site-directed mutagenesis of TraI active site tyrosine residues.
• Tracking TraI translocation during conjugative transfer.
• Analysis of TraI activity on various single-stranded DNA substrates.

Main Results:

• TraI is confirmed to be transported to the recipient cell during F plasmid transfer.
• Evidence suggests a second cleavage reaction occurs in the donor, generating a 3'-hydroxyl on the transferred ssDNA.
• Only Tyrosine 16 (Tyr16) in the TraI active site is critical for both cleavage reactions, implying two TraI molecules are needed.

Conclusions:

• Two TraI molecules are required for efficient F plasmid conjugative transfer: one for initial nicking and the second for facilitating transfer and re-circularization.
• TraI's slow dissociation from the 3'-end of the cleaved plasmid is likely essential for subsequent re-circularization.
• The findings provide a detailed mechanistic insight into the dual role of TraI in plasmid DNA metabolism.