Pilar Avila

Genes involved in conjugative DNA processing of plasmid R6K.

The conjugative transfer region of the IncX plasmid R6K (TRAX) was analysed by transposon mutagenesis and DNA sequencing. Tn5tac1 insertional mutations localized TRAX to a 14.8 kb segment containing the α origin of transfer (oriTα), genes involved in conjugative DNA‐processing (DtrX) and genes involved in pilus synthesis and assembly (MpfX). A second functional oriT, oriTβ, was located at a distance of 5.3 kb from oriTα and was outside TRAX. MpfX occupied a segment of 10 kb, as judged by the location of insertions conferring resistance to infection by the X pilus‐specific phage X‐2. At both sides of MpfX there were insertions that were Tra− but X‐2 sensitive, suggesting that the mutations were in DtrX. This region was sequenced and three genes were identified: taxA, taxB, and taxC. The overall organization was oriTα–taxA–taxC–MpfX–taxB. taxC coded for a oriT‐relaxase that belongs to the VirD2 family. taxA coded for a protein of 181 amino acids that showed similarity to TraY of F‐like plasmids and to the Arc‐repressor superfamily. TaxB showed similarity to TraG‐like proteins, a protein superfamily probably involved in coupling the relaxosome to the DNA‐transport apparatus. TaxA and TaxC are required for oriT nicking in vivo. The nicking reaction was mistakenly assumed by Flashner et al. (1996) to represent a feature of the vegetative replication origins. However, insertions or deletions disrupting taxA and taxC affected conjugation but not replication of R6K. Conversely, protein π, which is absolutely required for replication of R6K, was not required for conjugative transfer. In addition, protein DDP3, which is also assumed to have a role in replication, was found to be a positive modulator of bacterial conjugation. Taken together, these results rule out a direct and essential involvement of conjugation proteins in R6K vegetative replication, and also rule out the requirement of replication protein π for conjugation.

Plasmids containing one inverted repeat of Tn21 can fuse with other plasmids in the presence of Tn21 transposase

SummaryIn the presence of the Tn21 transposase, plasmids that contain a single Tn21 inverted repeat sequence fuse efficiently with other plasmids. This reaction occurs in recA strains, is independent of the transposon-encoded resolution system, and results in insertions into different sites in the recipient plasmid. All fusion products studied contained at least one complete copy of the donor plasmid; most also contained some duplication of it as well. The data are consistent with processive models of transposition.

Site-specific recombination and shuffling of resistance genes in transposon Tn21

Many multidrug-resistant transposons found in natural isolates of Gram-negative bacteria are close relatives of Tn21. Thus, the Tn21 subgroup of the Tn3 family of transposable elements is the most successful homogeneous group in acquiring resistance to newly introduced antibiotics. This paper summarizes the mode of acquisition of resistance genes by these elements.

Transposition-Like Events Mediated by Single-Ended Derivatives of Transposon Tn21

Transposable elements are specific sequences of DNA that can insert more or less at random into other DNA sequences. A “specific sequence” is defined by its ends and these are recognised during the transposition process. The ends of a particular element are usually identical or almost so and are thus called the “inverted repeats” (IR). During transposition, the IRs are recognised in a process that involves an element-encoded protein called “transposase”. (For a review of transposition, see ref.1.)