Reversion of a truncated gene for ampicillin resistance by genetic rearrangements in Escherichia coli K12

A pathway for the evolution of the plasmid NTP16 involving the novel kanamycin resistance transposon Tn4352

The kanamycin resistance determinant of the drug resistance plasmid NTP16 has been characterized by DNA sequencing and has been shown to possess all of the structural features of a transposable element. It is made up of a 1040-bp central region encoding a protein identical to the aminoglycoside 3'-phosphotransferase of Tn903, flanked by direct repeats of an element identical to IS26. This novel transposon has been designated Tn4352. Analysis of the host sequences flanking the transposon reveal that they are derived from a Tn3-like element, and contain no 8 base pair target size duplications which are normally created by the insertion of IS26-like elements. Comparison to the Tn3 sequence shows that the flanking sequences are noncontiguous within Tn3, with the clear implication that NTP16 has evolved from a similar plasmid encoding only ampicillin resistance (presumably NTP1) by the insertion of Tn4352 into the Tn3-like element, followed by a substantial deletion. The sequence analysis suggests that the initial insertion was into the tnpR gene of the ampicillin transposon, followed by a deletion extending to a specific site within tnpA.

Sequence relations among the IncY plasmid p15B, P1, and P7 prophages

Electron microscopic analysis of heteroduplex molecules between the 94-kb plasmid p15B and the 92-kb phage P1 genome revealed nine regions of nonhomology, eight substitutions, and two neighboring insertions. Overall, the homologous segments correspond to 83% of the P1 genome and 81% of p15B. Heteroduplex molecules between p15B and the 99-kb phage P7 genome showed nonhomology in eight of the same nine regions; in addition, two new nonhomologous segments are present and P7 carries a 5-kb insertion representing Tn902. The DNA homology between those two genomes amounts to 79% of P7 DNA and 83% of p15B. Plasmid p15B contains two stem-loop structures. One of them has no equivalent structure on P1 and P7 DNA. The other substitutes the invertible C segments of P1 and P7 and their flanking sequences including cin, the gene for the site-specific recombinase mediating inversion.