SUMMARY: DNA of the R plasmids RP1, RP4 and RP8 was isolated from various hosts. The lengths of these plasmid molecules were determined by electron microscopy: RP1 and RP4 were about 19 μm long, RP8 measured 31 μm. An RP4 plasmid mutant, designated RP4a, was isolated from ; it was about 1 μm shorter than normal RP4 DNA. To investigate the molecular relationship between RP4, RP4a and RP8, DNAs of these plasmids were partially denatured and examined in the electron microscope. Measurements of the length and denaturation pattern of the DNA molecules were used to construct physical maps. A new computer program was devised for the alignment of the circular molecules, and the effect of variations of different parameters on the reliability of the program was tested. A comparison of the denaturation pattern of RP4 and RP8 indicated that RP8 was composed of total RP4 plus an additional DNA fragment. The RP4a mutant plasmid could be defined as a deletion mutant with loss of 1 μm DNA.


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