Dissociation and Recombination of Fragments with Defined Functions of the Antibiotic Resistance Factor R1

Abstract

The antibiotic resistance factor R1drd-19 determines resistance to ampicillin (Ap), kanamycin (Km), sulfonamid (Sa), streptomycin (Sm) and chloramphenicol (Cm). It is derepressed in its transfer (drd) and occurs in the E. coli cell in few copies. As previously demonstrated by Sharp et al. /1/ R1 shares extensive sequence homologies with R6 and NR1 (R100-1). As shown in Table i most of the EcoRI fragments obtained with Rldrd -19 are, however, different from those of NR1 /2/. Nordström et al. /3/ have isolated several copy mutants (cop) of Rldrd-19, named Rldrd-19B2 (new nomenclature pKN) which have 3–10 fold increased copy numbers. One of them Rldrd-19B2 (pKN102) which does not seem to determine resistance to kanamycin has been studied in our laboratory and shown to give rise to the frequent generation of mini-plasmids, which we named Rsc plasmids /4/. Table 1 shows that Rldrd-19B2 differs from Rldrd-19 by the loss of three EcoRI fragments (F, J and O) and a slight alteration of EcoRI fragment D which carries the ampicillin resistance. The lost EcoRI fragments are present in a R1-derivative Rldrd-16, previously isolated by E. Meynell. This plasmid carries resistance to kanamycin but has lost the other four antibiotic resistances, determined by Rldrd-19 which apparently results in the loss of EcoRI-fragments D, G, H, I, L and N. Ridrd-16/1 is a kanamycin-sensitive mutant of Rldrd-16, which has been isolated in our laboratory. This deletion mutant which appears to be identical to the RTF unit of R1 again looses the three EcoRI-fragments F, J and 0, which are also absent in Rldrd-19B2.