Bacterial transposases and retroviral integrases

Transposable genetic elements have adopted two major strategies for their displacement from one site to another within and between genomes. One involves passage through an RNA intermediate prior to synthesis of a DNA copy while the other is limited uniquely to DNA intermediates. For both types of element, recombination reactions involved in integration are carried out by element-specific enzymes. These are called transposases in the case of DNA elements and integrases in the case of the best-characterized RNA elements, the retroviruses and retrotransposons. In spite of major differences between these two transposition strategies, one step in the process, that of insertion, appears to be chemically identical. Current evidence suggests that the similarities in integration mechanism are reflected in amino acid sequence similarities between the integrases and many transposases. These similarities are particularly marked in a region which is thought to form part of the active site, namely the DDE motif. In the light of these relationships, we attempt here to compare mechanistic aspects of retroviral integration with transposition of DNA elements and to summarize current understanding of the functional organization of integrases and transposases.