The two III fragments of polyoma virus DNA were cloned in the III site of plasmid pBR322, a site located in the RNA polymerase promoter involved in the expression of tetracycline resistance. Although insertion of foreign DNA into this site did not always result in the complete loss of tetracycline resistance, K12 strain 1776 harbouring recombinant plasmids exhibited reduced growth properties in liquid culture with tetracycline and could easily be differentiated from bacteria transformed by non-recombinant plasmids. The formation of plasmid multimers increased the resistance to tetracycline at the level of the induction period, presumably as a result of a gene dosage effect.


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