The invention relates to a method for deletion of antibiotic resistance and /or plasmid stabilisation. The invention includes the steps of constructing a vector comprising an antibiotic resistance gene surrounded by a direct repeat sequence gene. This direct repeat gene may be an essential gene or a Rek-sequence. In the latter case the essential gene with a suitable promoter is presented in the vector. A host cell is transformed with the vector obtained, followed by deletion of the essential chromosomal gene in the host cell and deletion of the antibiotic resistance gene in the vector in the cell. The essential gene infA is preferred.The invention also relates to a method of stable maintenance of a vector in a host cell, a method of producing DNA in the cell and a method of producing amino acids, preptides and proteins in the cell. Further, the invention is directed to transformed host cells from which the chromosomal essential gene has been deleted and which comprise a vector containing the corresponding essential gene and possibly also one or more genes X of interest. The vector carries no gene for antibiotic resistance. The use of vector DNA obtained from the host for the preparation of a pharmaceutical composition for gene therapy such as a vaccine is also covered. Bacteria carrying the vector with appropriate genes X can be used for large scale production of compounds as directed by the gene product(s) of such gene(s).
