Iterative selection from a Salmonella typhimurium cosmid library can lead to the isolation of an atypically small plasmid

We have constructed a DNA library from a virulent Salmonella typhimurium st rain, in an avirulent strain. The process of selecting the components of in terest from the library involved iterative growth in liquid culture. This r esulted; after four cycles: in the culture becoming homogeneous for a singl e plasmid, which was much smaller than the average size for the library. We have identified the larger precursor of this plasmid which has two regions of sufficient homology to allow recombination resulting in the formation o f the small plasmid. S. typhimurium carrying the small plasmid have a small er genetic burden than other members of the library and survive better in s pent culture medium, facilitating selection on repeated subculture. Such ra pid adventitious selection has important implications for isolation of clon es of interest from genomic libraries. (C) 1999 Federation of European Micr obiological Societies. Published by Elsevier Science B.V. All rights reserv ed.