HORIZONTAL GENE-TRANSFER

This review explores examples of horizontal genetic transfer in eukary otes and prokaryotes. The best understood of these involves various co nserved families of transposable elements, but examples of non-transpo sable-element-based movement of genes or gene clusters have also been identified in prokaryotic genomes. A unifying theme is the structural and DNA-sequence homology of transposable elements from widely unrelat ed genomes, suggesting evolutionarily conserved mechanisms for horizon tal transfer. This is reinforced by the fundamental similarity in the enzymatic mechanisms of retroviral integration (by integrases) and of transposition (by transposases). The review deals with various types o f horizontal transfer, the mechanisms available for such transfer, pot ential barriers, and the evolutionary significance of horizontal genet ic transfer.