Abstract: Methods for preparing nucleotide integrases are provided. The nucleotide integrases are prepared by combining in vitro an excised, group II intron RNA, referred to hereinafter as "exogenous RNA", with a group II intron-encoded protein. The exogenous RNA is prepared by in vitro transcription of a DNA molecule which comprises a group II intron sequence. In one embodiment, the group II intron-encoded protein is made by introducing into a host cell a DNA molecule that comprises at least the open reading frame sequence of a group II intron and then expressing the open reading frame sequence in the host cell. The DNA molecule may comprise the open reading frame sequence operably linked to a promoter, preferably an inducible promoter. Thereafter, the cell is fractionated and the protein is recovered and combined in vitro with the exogenous RNA to provide RNP particles having nucleotide integrase activity.