Abstract

R483, a plasmid of the Ialpha incompatibility group, contained a deoxyribonucleic acid (DNA) sequence encoding resistance to trimethoprim (TpR) and streptomycin (SmR) that could be transposed to other replicons, i.e., to the Escherichia coli chromosome and to related and unrelated plasmids. Each transposition resulted in the acquisition by the recipient replicon of a segment of DNA of about 9 X 10(6) daltons, both resistance genes, but never the colicin Ia or pilus genes of R483. Transposition took place at a single chromosomal site between dnaA and ilv and did not suppress the DnaA phenotype, in contrast to integration of the whole R483 plasmid. The chromosome, having received the transposition, could secondarily act as a transposition donor to another plasmid. Such a plasmid was indistinguishable from one having received a direct transposition from R483. TpR SmR transposition was very site specific and did not require a functional recA+ gene. We postulate that the TpR SmR segment of R483 is a transposon (TnC) with specific boundary sequences.