Disappearance of the covalently closed circular cryptic plasmid PO-2 in Salmonella pullorum MS35 containing F77
The physical basis of the donor property of Salmonella pullorum donor strains MS8300, MS830, and MS831 carrying the F77 factor from Salmonella typhimurium was investigated by dye-buoyant density equilibrium centrifugation and zonal centrifugation in neutral sucrose gradients. Centrifugation of the MS8300 and MS831 closed circular DNA material in a 20 to 31% neutral sucrose gradient resulted in a profile having one sharp peak of radioactivity with a sedimentation coefficient of 17 S and a broad peak extending from 65 to 70 S. The 17- and 65-S species were isolated from the isogenic F− strain MS83. These appeared identical with those isolated previously in S. pullorum MS53 as the cryptic plasmids PO-1 and PO-2 respectively. Cosedimentation of differentially labeled F77 DNA and the lysate containing the 65-S and 70-S species suggested that the 70-S species is the autonomous F77 factor in strains MS8300 and MS831. Lysates of MS830 similarly treated produced a profile containing the 17-S molecule and possibly some 70-S molecules but no 65-S molecules. It was concluded that the F77 factor was integrated in most cells and that the covalently closed circular state of PO-2 plasmid was lost. The mutation in the cysE gene of the F77 factor carried by MS831 had no apparent effect on the covalently closed circular nature of PO-2 plasmid, although F77 no longer seemed to mobilize the chromosome from the cysE locus.