TRANSITORY DEREPRESSION AND THE MAINTENANCE OF CONJUGATIVE PLASMIDS

Peter D. Lundquist ¹ and Bruce R. Levin ¹

¹ Department of Zoology, University of Massachusetts, Amherst, Massachusetts 01003

It has been proposed that bacterial plasmids cannot be maintained by infectious transfer alone and that their persistence requires positive selection for plasmid-borne genes. To test this hypothesis, the population dynamics of two laboratory and five naturally occurring conjugative plasmids were examined in chemostat cultures of E. coli K-12. Both laboratory plasmids and three of the five wild plasmids failed to increase in frequency when introduced at low frequencies. However, two of the naturally occurring plasmids rapidly increased in frequency, and bacteria carrying them achieved dominance in the absence of selection for known plasmid-borne genes. Three hypotheses for the invasion and persistence of these two plasmids were examined. It is concluded that although these two extrachromsomal genetic elements are repressed for conjugative pili synthesis, as a consequence of high rates of transfer during periods of transitory derepression in newly formed transconjugants, they become established and are maintained by infectious transfer alone. The implications of these observations to the theory of plasmid maintenance and the evolution of repressible conjugative pili synthesis are discussed.

This article has been cited by other articles:

				S. M. Krone, R. Lu, R. Fox, H. Suzuki, and E. M. Top Modelling the spatial dynamics of plasmid transfer and persistence Microbiology, August 1, 2007; 153(8): 2803 - 2816. [Abstract] [Full Text] [PDF]

				P. E. Turner Phenotypic Plasticity in Bacterial Plasmids Genetics, May 1, 2004; 167(1): 9 - 20. [Abstract] [Full Text] [PDF]

				J. Paulsson Multileveled Selection on Plasmid Replication Genetics, August 1, 2002; 161(4): 1373 - 1384. [Abstract] [Full Text] [PDF]

				C. T. Bergstrom, M. Lipsitch, and B. R. Levin Natural Selection, Infectious Transfer and the Existence Conditions for Bacterial Plasmids Genetics, August 1, 2000; 155(4): 1505 - 1519. [Abstract] [Full Text]