Immunity of replicating Mu to self-integration: a novel mechanism employing MuB protein

Jun Ge,Rasika M Harshey,Zheng Lou

doi:10.1186/1759-8753-1-8

Abstract

We describe a new immunity mechanism that protects actively replicating/transposing Mu from self-integration. We show that this mechanism is distinct from the established cis-immunity mechanism, which operates by removal of MuB protein from DNA adjacent to Mu ends. MuB normally promotes integration into DNA to which it is bound, hence its removal prevents use of this DNA as target. Contrary to what might be expected from a cis-immunity mechanism, strong binding of MuB was observed throughout the Mu genome. We also show that the cis-immunity mechanism is apparently functional outside Mu ends, but that the level of protection offered by this mechanism is insufficient to explain the protection seen inside Mu. Thus, both strong binding of MuB inside and poor immunity outside Mu testify to a mechanism of immunity distinct from cis-immunity, which we call 'Mu genome immunity'. MuB has the potential to coat the Mu genome and prevent auto-integration as previously observed in vitro on synthetic A/T-only DNA, where strong MuB binding occluded the entire bound region from Mu insertions. The existence of two rival immunity mechanisms within and outside the Mu genome, both employing MuB, suggests that the replicating Mu genome must be segregated into an independent chromosomal domain. We propose a model for how formation of a 'Mu domain' may be aided by specific Mu sequences and nucleoid-associated proteins, promoting polymerization of MuB on the genome to form a barrier against self-integration.

Highlights

Transposition is a double-edged sword, allowing elements to populate new sites within their host genomes while potentially exposing their own DNA to self-disruption
This method will detect insertions whose R end is oriented towards the L end of the recipient in which packaging initiates; the R end of an oppositely oriented insertion would lie beyond the head-full size of ~40 kb
Some bias has been reported at the local level [24,25], Mu insertions in the Eschericia coli genome generally show no orientation bias at the level of W/C strands; we believe that this is true of insertions within Mu as well and that the data are representative of all insertions

Summary

Introduction

Transposition is a double-edged sword, allowing elements to populate new sites within their host genomes while potentially exposing their own DNA to self-disruption. Several bacterial transposons including members of the Tn3 family, Tn7 and bacteriophage Mu display transposition immunity [1]. These elements avoid insertion into DNA molecules that already contain a copy of the transposon (a phenomenon called cisimmunity) and it is thought that this form of self-recognition must provide protection against self-integration. Cis immunity does not provide protection to the whole bacterial genome on which the transposon is resident, but can extend over large distances from the chromosomal site where the transposon is located, or over an entire plasmid harboring the transposon.

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