The invention provides cells and methods of circularizing linear DNA molecules. The cell is an isolated Escherichia coli cell which transiently expresses the Cre recombinase protein from an integrated Cre recombinase gene, and which is at least transiently repressed for RecBCD activity. The cells are used in a method of circularizing a linear DNA molecule comprising at least two loxP sites. The DNA molecule is introduced into the cells, and the linear DNA molecule is joined at said loxp sites.