Interaction between R-factors and the host cells. I. Lethal interference between R-factor determined streptomycin resistance and streptomycin dependence in Proteus mirabilis

Direct selection for the exchange of alleles between a plasmid and the Escherichia coli chromosome

Recombination is extensively used in order to move alleles between replicons. The exchange of wild-type chromosomal and mutant plasmid-borne alleles is a two-step process entailing the formation of a cointegrate between the entire plasmid and the chromosome, followed by resolution of such cointegrates to give a mutant chromosome and a plasmid carrying the wild-type chromosomal sequence. Often the cointegrate and the resolved forms cannot be distinguished phenotypically. To enable the direct isolation of the resolved products we have developed a positive selection technique. Cells containing a cointegrated plasmid R1 were constructed by transduction using a P1 lysate prepared from cells harbouring a plasmid comprising a mutant chromosomal allele and the so-called omega fragment which carries an aad (aminoglycoside adenylyltransferase) gene. P1 transduction from the cointegrate strain into an SmD recipient allowed direct selection for the resolved complex, since transduction of the aad gene is lethal to an SmD strain.

Maintenance of bacterial plasmids: comparison of theoretical calculations and experiments with plasmid R1 in Escherichia coli

Plasmid R1 was tested for stable maintenance in Escherichia coli K12. Populations carrying a transfer-negative derivative of plasmid R1drd-19 were grown exponentially for 72 generations in LB medium. Out of nearly 5000 cells none had lost the plasmid; hence the rate of loss of the plasmid is less than 3 X 10(-6) per cell and cell generation. Other experiments showed that the loss rate was less than or equal to 10(-7) per cell and cell generation. Plasmid R1 replication is controlled so that, on average, n copies are synthesized per cell and cell generation irrespective of the copy number (as long as it is at least one). A theoretical analysis was performed, based on the assumption that there is a Poissonian spread around n in each class of cells (each class defined by the number of R1 copies at birth). Two models for partitioning were analyzed, Equi-partitioning and Pair Site Partitioning; in the latter, each daughter is guaranteed one plasmid copy, whereas the rest of the copies are assumed to be randomly distributed. The copy number distribution was found to be fairly narrow in both cases, with at least 90% of the population in the range n/2-3n/2 for baby cells and in the range n-3n for cells just before cell division. Plasmid-free cells were estimated to appear at a frequency which for the copy number of R1 (n = 3-4) was 1.5 X 10(-3) - 8 X 10(-5) for equipartitioning and 6 X 10(-3) - 6 X 10(-4) for pair site partitioning, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Salmonella typhimurium LT2 mutation affecting the deletion of resistance determinants on R plasmids

Plasmid Rms312, specifying resistance to tetracycline (Tc), chloramphenicol (Cm), streptomycin (Sm), sulfonamide (Su), and mercury chloride (Mer), deletes both Tc and Cm Sm Su Mer determinants at a high frequency in Salmonella typhimurium LT2. S. typhimurium mutants that were stable carriers of Rms312 were isolated by alternate culture of R-bearing cells in a medium containing either tetracycline or chloramphenicol. In one of these mutants the deletion frequency of drug resistance determinants was decreased by about 100-fold not only Rms312, but also in R100, R1, and R6-5. This mutation caused a slight reduction of ultraviolet resistance but did not affect generalized genetic recombination, indicating that the mutation is different from recA. The mutation, designated dor (deletion of r-determinants), was mapped to a position near 57 units in the new linkage map of S. typhimurijm LT2 (K. E. Sanderson and P. E. Hartman, Microbiol. Rev. 42:471-519, 1978). The dor mutation had no effect on IS1-mediated illegitimate deletion, indicating that the dor mutation is different from the del mutation described by Nevers and Saedler (P. Nevers and H. Saedler, Mol. Gen. Genet. 160:209-214, 1978).