High-frequency conjugation of the lactose plasmid pLP712 is associated with a constitutive cell aggregation phenotype and is facilitated by cointegration with a sex factor. Analysis of 23 independently derived enlarged lactose plasmids revealed that the sex factor DNA present in cointegrates varied in size. This suggested that more than simple cointegration with a sex factor plasmid was involved. Further analysis led to the discovery of a chromosomally located sex factor that could excise and be lost or exist as labile plasmid DNA. Cointegration with this sex factor was shown to be promoted by transposition of a copy of ISSI present on the lactose plasmid, and models are presented to account for the complex and variable structures of the resulting enlarged lactose plasmids.
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