USE OF THE REP TECHNIQUE FOR ALLELE REPLACEMENT TO CONSTRUCT NEW ESCHERICHIA-COLI HOSTS FOR MAINTENANCE OF R6K-GAMMA ORIGIN PLASMIDS AT DIFFERENT COPY NUMBERS

Escherichia coli hosts were constructed for maintenance of vectors con taining the gamma replication origin of the R6K plasmid (oriR(R6K gamm a)) at different copy numbers (15 or 250/cell). Such vectors require t he trans-acting II protein (the pir gene product) for replication. New hosts carry pir(+) or pir -116 on the chromosome within uidA, the E. coli gene encoding beta-glucuronidase. They were made using the rep te chnique for allele replacement and Km(R) M13 Delta uidA::pir(+) or M13 Delta uidA::pir-116 phage. Because M13 cannot replicate in a rep muta nt, Km(R) transductants arose by integration into the chromosomal uidA locus. Segregants lacking M13 sequences (which were selected as deoxy cholate-resistant (DoC(R)) ones) frequently contained Delta uidA::pir( +) or Delta uidA::pir-116 on the chromosome. In principle, this proced ure could be used for the introduction of any foreign gene into any no nessential gene on the E. coli chromosome. The Delta uidA::pir(+) and Delta uidA::pir-116 loci were subsequently transferred to a variety of E. coli strains. One such strain is a suppressor-negative one that is especially useful for transposon (Tn) mutagenesis. This strain has an integrated RP4 derivative for conjugative transfer of oriR(R6K7) plas mids also containing oriT from RP4. In addition, new oriR(R6K gamma), oriT(+) vectors carrying the Tc-R- encoding genes tetAR from Tn10 are described. These can be used for allele replacement by conjugative tra nsfer of an oriR(R6K gamma), oriT(+), tetAR plasmid containing a mutat ed gene into a non-pir recipient and by subsequent selection for Tc-se nsitive exconjugants.