IS8- and Tn2353-mediated cointegration of the plasmids R15 and RP4:: Tn1

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Abstract
The plasmids R15 and RP4:: Tn1 form fused structures (85 Md and 92 Md cointegrates). The cointegrates do not resolve practically in recA − Escherichia coli cells and have a mean life-time of more than 50 generations in a recA + background. The 85 Md cointegrates were generated at a frequency of 4×10−4 per R15 transconjugant during a mating between E. coli [R15; RP4:: Tn1] and E. coli [F′ColVBtrp:: Tn1755]. These plasmids carry two directly repeated copies of the mobile element IS8 at the junctions between R15 and RP4:: Tn1. The transposition of IS8 from RP4:: Tn1 to the R15 plasmid and the formation of hybrid molecules promoted by this process appear to be induced by the IS8 element of the Tn1755 structure during or after conjugal transfer of F′ColVBtrp:: Tn1755 into E. coli [R15; RP4:: Tn1] cells. The formation of the 92 Md cointegrates occurs at a frequency of 2×10−5. The fused molecules of R15 and RP4:: Tn1 carry two direct copies of an 8.65 Md R15 fragment at the junctions between these replicons. The fragment has specific features of a new transposon. This element designated Tn2353 determines resistance to Hg, Sm and Su and contains two sites for each BamHI, BglII and SalI and three sites for both EcoRI and PstI. The physical map and some other characteristics of Tn2353 are presented.