# Cryo-EM structure of an asymmetric TraI dimer shows how F-plasmid conjugation is initiated

- Published: 2026-11-05 02:09 UTC
- Section: Life & Health › Biology & biochemistry
- Tags: bacterial conjugation, TraI relaxase-helicase, F-plasmid, cryo-EM structure, horizontal gene transfer, antibiotic resistance, trans-esterase, IHF host factor, origin of transfer
- URL: https://realtech.media/news/rt-ca227152c7ac123690f5b196_cryo_em_structure/

**Researchers from Queen Mary University of London and Imperial College London resolved the TraI homodimer: one molecule melts DNA at the transfer origin, the other loads the helicase.**

![Cryo-EM structure of an asymmetric TraI dimer shows how F-plasmid conjugation is initiated](https://sgzwpqqelzmajkcubgkl.supabase.co/storage/v1/object/public/rt-media/2026/11/bebada2f1a23f30d.jpg)
*Tadeáš Bednarz, Wikimedia Commons, CC BY-SA 4.0*

Researchers at Queen Mary University of London and Imperial College London have reported the cryo-EM structure of a functionally asymmetric TraI homodimer, the relaxase-helicase of the F-plasmid conjugation system. The paper by Danylo Gorenkin, Ruina Liang, Jonasz B. Patkowski, Salomé Bodet-Lefèvre, Tiago R. D. Costa and Aravindan Ilangovan appeared in Nature Communications on 3 November 2026 [1].

## TraI and the start of conjugation

Bacterial conjugation drives horizontal gene transfer and accelerates the spread of antibiotic resistance. In F-plasmid systems, TraI is a bifunctional enzyme. It initiates transfer by nicking plasmid DNA through its trans-esterase activity, and then unwinds the plasmid with its helicase domains [1].

Previous evidence suggested that two TraI molecules act cooperatively. How their activities are coordinated in space and time had remained unclear [1].

## Asymmetric dimer and control by IHF

In the reported structure, the two TraI molecules play different roles. According to the authors, dimerisation is mediated predominantly by single-stranded DNA [1].

One TraI molecule melts the DNA duplex at the origin of transfer by means of its trans-esterase activity. This melting enables a second TraI molecule to load its helicase onto the transfer strand of the intact plasmid [1].

The trans-esterase domain is intrinsically active. The host factor IHF, however, inhibits nicking until helicase loading occurs, the authors report [1].

The authors describe this as a regulatory loop that couples strand nicking with helicase engagement. They present it as a unified mechanism for initiating conjugative DNA processing [1].

The data thus place two activities of a single protein, nicking and unwinding, in an ordered sequence. In that sequence the nicking step is held back by IHF until the partner molecule has loaded the helicase [1].

## Sources

1. [Nature Communications](https://doi.org/10.1038/s41467-026-78888-w)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-ca227152c7ac123690f5b196_cryo_em_structure/