Transposition of DNA fragments flanked by two inverted Tn1 sequences: translocation of the plasmid RP4::Tn1 region harboring the Tcr marker

Physical and genetic structure of the IncN plasmid R15

Restriction sites for seven hexanucleotide-specific endonucleases were located on the map of the conjugative IncN plasmid R15 (SmrSurHgr, 62.3 kb). The distribution of the cleavage sites is strongly asymmetric. Twenty-eight of thirty-four sites for BamHI, EcoRI, HindIII, SalI, SmaI, and PstI were located close to or within the sequences of an IS5-like element and the transposons Tn2353 and Tn2354. By analysis of R15::Tn1756 deletion derivatives and recombinant plasmids harboring R15 fragments, the genetic determinants for the streptomycin, sulfonamide, and mercury resistances were mapped, as well as the regions necessary for EcoRII restriction-modification and for plasmid replication and conjugation. The features of physical and genetic structures of the plasmid R15 and other IncN plasmids are discussed.

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

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 x 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 x 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, Bg/II and Sa/I and three sites for both EcoRI and PstI. The physical map and some other characteristics of Tn2353 are presented.

Gene regulation in plasmid RK2: positive control by korA in the expression of korC

The broad-host-range plasmid RK2 encodes three host-lethal kil genes whose actions are controlled by specific kor genes. We have shown previously that the 0' to 5.5' region of RK2 encodes both kilA and korC. Because of the lethal effect of kilA, plasmids with this region cannot be maintained in Escherichia coli unless the RK2 korA gene is also present. To investigate korC in the absence of kilA and therefore of korA, we first mapped kilA and korC to specific segments of the cloned 0' to 5.5' region. This allowed us to construct a korC+ plasmid missing the kilA region and thereby removed the need to have korA in the cell. We found that this korC-encoding plasmid alone is insufficient to control kilC. The korA function is required, and it can be supplied in trans. We also constructed a kilA+ korC- plasmid and found that korA is sufficient to control kilA. Thus, in addition to acting negatively to control kilA, korA acts positively to allow korC control of kilC. This korA dependence of korC is bypassed in a rho-115 mutant of E. coli. We consider the possibility that korA product acts as an antiterminator of transcription in korC expression.