J. Grinsted

The Tn21 subgroup of bacterial transposable elements.

The Tn3 family of transposable elements is probably the most successful group of mobile DNA elements in bacteria: there are many different but related members and they are widely distributed in gram-negative and gram-positive bacteria. The Tn21 subgroup of the Tn3 family contains closely related elements that provide most of the currently known variation in Tn3-like elements in gram-negative bacteria and that are largely responsible for the problem of multiple resistance to antibiotics in these organisms. This paper reviews the structure, the mechanism of transposition, the mode of acquisition of accessory genes, and the evolution of these elements.

Transposition of TnA Does Not Generate Deletions

SummaryWe have examined the incidence of loss of the TnA unit, Tn801, from RP1 under conditions where transposition of Tn801 to another replicon, R388, was readily detected. We found that the frequency of transposition of Tn801 from RP1 to R388 exceeded, by at least a factor of one hundred, the frequency at which it was deleted from RP1. We conclude that, in general, transposition of Tn801 does not generate derivatives of the donor plasmid which specifically lack Tn801. The relevance of these findings to the mechanism of transposition is discussed.

Physical and genetic analysis of the Inc-W group plasmids R388, Sa, and R7K.

Abstract Three Inc-W group plasmids R388, Sa, and R7K have been studied by a variety of physical and genetic techniques. The data presented here permit the mapping of function onto the restriction enzyme maps of all three plasmids. When the physical and functional maps of these three plasmids are then compared they show a high degree of sequence conservation in the regions encoding replication and transfer functions but large differences in the regions which code for antibiotic resistance. In all three plasmids the DNA in and around the antibiotic resistance genes contains the majority of the restriction enzyme sites. The evolution of these three plasmids is discussed.

Characterisation of Tn501, a transposon determining resistance to mercuric ions

SummaryThe transposon encoding resistance to mercuric ions, Tn501, is 5.2 (±0.1)×106 daltons and is bounded by small inverted repeats. The restriction sites for the restriction endonucleases EcoRI, HindIII and SalGI have been mapped on the element.

DNA sequences of and complementation by the tnpR genes of Tn21, Tn501 and Tn1721

SummaryDNA sequences that encode the tnpR genes and internal resolution (res) sites of transposons Tn21 and Tn501, and the res site and the start of the tnpR gene of Tn1721 have been determined. There is considerable homology between all three sequences. The homology between Tn21 and Tn501 extends further than that between Tn1721 and Tn501 (or Tn21), but in the homologous regions, Tn1721 is 93% homologous with Tn501, while Tn21 is only 72–73% homologous. The tnpR genes of Tn21 and Tn501 encode proteins of 186 amino acids which show homology with the tnpR gene product of Tn3 and with other enzymes that carry out site-specific recombination. However, in all three transposons, and in contrast to Tn3, the tnpR gene is transcribed towards tnpA gene, and the res site is upstream of both. The res site of Tn3 shows no obvious homology with the res regions of these three transposons. Just upstream of the tnpR gene and within the region that displays common homology between the three elements, there is a 50 bp deletion in Tn21, compared to the other two clements. A TnpR− derivative of Tn21 was complemented by Tn21, Tn501 and Tn1721, but not by Tn3.

Regional preference of insertion of Tn501 and Tn802 into RP1 and its derivatives

SummaryThe sites of insertion of Tn501 into RP1 and into derivatives of this plasmid that either lack the Tn801 (TnA) element or contain it in a different location have been determined. Similarly, the sites of insertion of Tn802 into a derivative of RP1 that lacks the Tn801 element and into recombinants of this plasmid with Tn501 were determined. ‘Hot spots’ for insertion were observed with both transposons; but it is clear that a particular DNA sequence is not sufficient to define a ‘hot spot’, since a particular region does contain many insertions when present in one plasmid but does not do so when part of another.

Complementation of transposition of tnpA mutants of Tn3, Tn21, Tn501, and Tn1721

Abstract The transposons Tn21, Tn501, and Tn1721 are related to Tn3. Transposition-deficient mutants (tnpA) of these elements were used to test for complementation of transpostion. Transposition of tnpA mutants of Tn501 and Tn1721 was restored by the presence in trans of Tn21, Tn501, and Tn1721, but transposition of a tnpA mutant of Tn21 was restored in trans only by Tn21 itself. Tn3 did not complement transposition of Tn21, Tn501, or Tn1721, and these elements did not complement transposition of Tn3.

A restriction enzyme map of R-plasmid RP1

Abstract The relative positions of the sites on RP1 of the following restriction enzymes were mapped: EcoRl, BamH1, HindIII, BglII, SmaI, and PstI.

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.

A Tn21 terminal sequence within Tn501: complementation of tnpA gene function and transposon evolution

SummaryThe prokaryotic mercury-resistance transposon Tn501 contains a sequence, 80 nucleotides from one end, which is identical with an inverted terminal repeat (IR) of Tn21. This Tn21 IR sequence is used when Tn21 complements a TnpA- derivative of Tn501, but not when Tn501 is used for the complementation. Complementation by Tn1721 shows a preference for the normal Tn501 IRs. The element (Tn820) transposed when Tn21 is used to complement a Hg- TnpR- TnpA- Res- deletion mutant of Tn501 contains the Tn21 IR sequence at one terminus and a Tn501 IR at the other. Transposition of Tn820 can be complemented by Tn501 and Tn1721, but at a much lower frequency than transposition of the parental element (Tn819) which has two Tn501 IRs. The relationship between the transposition functions of Tn501, Tn21 and Tn1721, and available nucleotide sequence data suggest that Tn501 evolved by the transposition of a Tn21-like element into another transposable element (similar to that found within Tn1721) followed by deletion of the Tn21-like transposition functions.