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Curing of F-like plasmid TP181 by plumbagin is due to interference with both replication and maintenance functions

School of Biological Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
Department of Microbiology, Sri Padmavathi Mahila Viswavidyalayem, Tirupati-517 507 (AP) India

³Author for correspondence: Christopher M. Thomas. Tel: +44 121 414 5903. Fax: +44 121 414 5925 e-mail: C.M.Thomas@bham.ac.uk

ABSTRACT

F-like plasmid TP181 is particularly susceptible to curing by the naphthoquinone derivative plumbagin, which may attack DNA gyrase. TP181 should provide particular insight into the basis of plasmid elimination, which has application in a number of contexts. Curing was found to be optimal at pH 7.2. An EcoRl fragment containing the RF1A replicon of TP181 was joined to a Km^R determinant (giving miniTP181). MiniTP181 had the same increased susceptibility to curing by plumbagin when compared to miniF as TP181 had relative to F. Plumbagin interfered with replication of miniTP181, depressing its copy number and increasing the rate of segregation. Plumbagin also blocked the lethal effect of the ccd locus after rifampicin treatment, which mimics production of plasmid-free segregants, so that more of these plasmid-free cells would survive. Restriction mapping and DNA sequence analysis indicated that the ccd locus of TP181 is almost identical to that of F but that TP181 lacks the repC gene present in F which is needed to activate replication from oriV. Thus the sensitivity of TP181 may be due to its dependence on both a replicon which is hypersensitive to perturbation of supercoiling by plumbagin and a host-killing system which is blocked by plumbagin.