A SITE-SPECIFIC DNA INVERSION IN BACTEROIDES PLASMID-PBF4 IS INFLUENCED BY THE PRESENCE OF THE CONJUGAL TETRACYCLINE RESISTANCE ELEMENT

pBF4 is a 42-kb R plasmid from Bacteroides fragilis which transfers clindamycin resistance (Cl(r)) independently of the chromosomal tetracycline resistance (Tc(r)) transfer element. We have found that this plasmid exists in two nonequimolar conformations, A and B. These forms differ by an inversion of approximately 11.5 kb which does not involve the repeated DNA sequences previously mapped on the plasmid. The presence of chromosomal tetracycline resistance conjugal elements influences the relative amounts of the two conformations: induction with tetracycline shifts the dominant form from B to A.