Evidence for a composite state of an F'his, gnd element and a cryptic plasmid in a derivative of Salmonella typhimurium LT2

Physical characterization of a plasmid cointegrate containing an F'his gnd element and the Salmonella typhimurium LT2 cryptic plasmid

A recombinant plasmid (pAS19) isolated from a derivative of Salmonella typhimurium LT2, containing the strain LT2 cryptic plasmid and an F'his gnd element, has been physically characterized. The pAS19 plasmid contour length equals the sum of the contour lengths of the cryptic plasmid and F'his gnd element. Deoxyribonucleic acid (DNA)-DNA hybridization experiments demonstrated that whereas the pAS19 plasmid exhibits extensive DNA homology with both the cryptic plasmid and the F'his gnd element, there is little DNA homology between these latter two plasmids. The DNA fragmentation pattern of the pAS19 plasmid produced by the restriction endonuclease R-EcoRI is consistent with that expected for a composite plasmid cointegrate containing most, if not all, of the DNA sequences present in its two component plasmids.

Genetic and physical studies of recombinant plasmids formed between an R plasmid of compatibility group FI and sex factor F of HfrH

Recombinant plasmids between an R plasmid of the FI group (R162/3) and the sex factor F or HfrH were produced after the conjugal transfer of this R plasmid into HfrH. Three types of recombinant plasmids were identified after the mating of HfrH (R162/3) with recA and rec+ recipients. One specimen of each type (pIP218, pIP222, pIP226) was studied in this report. All three recombinant plasmids carry the same genetic information for resistance to antibiotics (CSSuT) retained from R162/3. pIP218 retained all the other properties from F of HfrH: derepression for pilus synthesis, mobilization of the chromosome for the proximally transferred HfrH genes (thr, leu, proA), interference with T7 propagation, and ability to be cured by acridine orange. pIP222 retained from F of HfrH the derepression for pilus synthesis and the same polarity of chromosome transfer (thr, leu, proA), while pIP226 retained the interference with T7 propagation and acridine orange curing. Physical studies revealed that replication control and/or recovery of F and pIP218 as covalent circles of deoxyribonucleic acid are similar, and are different from R162/3. The new plasmids are more likely the result of a substitutive recombination event than a fusion. We propose genetic maps of these recombinant plasmids, showing the unequal participation of the parental plasmids in their formation.

Recombinant plasmid obtained from two different, compatible staphylococcal plasmids

From two different, compatible staphylococcal plasmids that determine streptomycin and chloramphenicol resistance, respectively, a recombinant plasmid was obtained. This plasmid can be transduced with a rather high frequency (10(-4)/plaque-forming unit) to plasmid-negative strains, the linkage of the two markers being 100%. The maintenance of the recombinant plasmid in the host cell seems to be controlled by the chloramphenicol resistance plasmid. The recombinant plasmid proved to be incompatible with both parental plasmids, which are unrelated. The relationship between the chloramphenicol resistance plasmid and the recombinant plasmid was the same as the between genetically marked derivatives of the recombinant plasmid, whereas the relationship of the streptomycin resistance plasmid to the recombinant plasmid was of a different, asymmetrical type.

Integrative suppression of a dnaA mutation in Salmonella typhimurium

Integrative suppression of a dnaA mutation in Salmonella typhimurium may result from the integration of F'lac or F'his into the chromosome in the left hand side of the chromosomal map. The suppressed revertants resulting from this integration do not contain DNA of the F' elements in the covalently closed circular (CCC)1 form but still contain the CCC DNA of the cryptic LT2 plasmid. Two suppressed revertants isolated from dnaA/F- strains were found in which the suppression of dnaA character was accompanied by the loss of CCC DNA from the cell lysates. From one of these revertants a segregant was isolated in which the return to the dnaA phenotype was accompanied by the reappearance of CCC DNA in the cell lysate. It is suggested that the cryptic plasmid may integrate into the chromosome of S. typhimurium and this integration may result in suppression of the dnaA mutation. Additional evidence suggesting that the cryptic plasmid controls its own initiation of replication independently of the function of the chromosomal dnaA gene is supplied by the results of the determination of incorporation of labelled thymidine into CCC DNA of the dnaA1 strain at the nonpermissive temperature.