title: Molecular recombination between R-factor deoxyribonucleic acid molecules in Escherichia coli host cells.
abstract: THREE PREVIOUSLY STUDIED R FACTORS WERE USED: 222/R4, controlling transmissible resistance to sulfonamide, streptomycin, chloromycetin, and tetracycline (SU(r) SM(r) CM(r) TC(r)); 222/R3, a derivative of 222/R4 (now termed 222/R3W) having lost TC(r); and R15, controlling infectious resistance to SU and SM only. Two types of derivative R factors were isolated from 222/R4 by serial subculture in Salmonella species. One derivative, termed 222/R1, lost resistance to SU, SM, and CM, and the other, termed 222/R3N, lost only TC(r). Each factor was transferred to a standard Escherichia coli K-12 host. Recombinant factors of 222/R4 phenotype were isolated by selection after mixed culture of E. coli (222/R1)(+) and (222/R3N)(+) strains. Density-gradient equilibrium centrifugation of lysates of E. coli R(+) hosts in the presence of ethidium bromide separated R-factor deoxyribonucleic acid (DNA) as a heavy satellite peak which was subjected to electron microscopy or analytical density gradient centrifugation. Each DNA comprised a unimolecular species of circular DNA. The contour of R15 measured 22.3 mum [equivalent to 46 x 10(6) atomic mass units (AMU)], and that of 222/R4 measured 33.6 mum (70 x 10(6) AMU). 222/R3W appeared to be a point mutant or small deletion of 222/R4 with an almost identical size, whereas 222/R3N had lost a DNA segment of about 3 mum, and measured 30.3 mum or 63 x 10(6) AMU. The 222/R1 factors also appeared to have arisen by loss of DNA from 222/R4, 222/R1A being 22.3 mum or 46 x 10(6) AMU, whereas all other 222/R1 factors appeared to be duplicates, measuring 25.6 mum or 53 x 10(6) AMU. The DNA from six recombinant factors of R4 phenotype was indistinguishable in size and configuration from the parental 222/R4. In most cases, the number of R-factor copies (present as covalently closed circular molecules) per copy of the E. coli chromosome was less than 2, ranging from 1.2 to 3.3.
mesh_terms: D002499:Centrifugation, Density Gradient; D002701:Chloramphenicol / Q000494:pharmacology; D002876:Chromosomes, Bacterial; D004269:DNA, Bacterial / Q000302:isolation & purification; D004352:Drug Resistance, Microbial; D004926:Escherichia coli / Q000187:drug effects; D005827:Genetics, Microbial; D008854:Microscopy, Electron; D008962:Models, Theoretical; D010617:Phenanthridines; D010636:Phenols; D011511:Proteus; D011868:Radioisotopes; D011995:Recombination, Genetic; D012486:Salmonella typhimurium; D013307:Streptomycin / Q000494:pharmacology; D013449:Sulfonamides / Q000494:pharmacology; D013752:Tetracycline / Q000494:pharmacology; D014025:Titanium
chemical_list: D004269:DNA, Bacterial; D010617:Phenanthridines; D010636:Phenols; D011868:Radioisotopes; D013449:Sulfonamides; D002701:Chloramphenicol; D014025:Titanium; D013752:Tetracycline; D013307:Streptomycin