Architecture of the Tn7 posttransposition complex: an elaborate nucleoprotein structure

Area of Science:

• Molecular Biology
• Genetics
• Bacterial Transposons

Background:

• The bacterial transposon Tn7 undergoes site- and orientation-specific insertion into the attTn7 chromosomal site.
• Transposition is mediated by four proteins: TnsA, TnsB, TnsC, and TnsD.

Purpose of the Study:

• To identify the specific Tns proteins constituting the active transpososome.
• To elucidate the protein composition of the posttransposition complex (PTC).

Main Methods:

• Utilized fluorescently labeled Tns proteins to determine their presence in the PTC.
• Employed high-resolution DNA footprinting to analyze DNA-protein interactions.

Main Results:

• All four Tns proteins (TnsA, TnsB, TnsC, TnsD) are present in the PTC.
• Protein-protein contacts between Tns proteins were found to pair the Tn7 ends.
• Oligomers of TnsA, TnsB, and TnsC likely mediate transposition.
• Extensive DNA footprinting revealed ~350 bp of DNA at Tn7 ends and attTn7 are protected by Tns proteins.
• TnsB binding sites are occupied, with altered protection patterns suggesting end pairing and target complex interaction.

Conclusions:

• The PTC contains all four Tn7 transposition proteins, indicating their roles in the transposition process.
• Tn7 end pairing and interaction with the attTn7 target site are crucial for transposition.
• TnsD imposes alternative DNA structures on attTn7 that are maintained in the PTC.