Tn552, one of several closely related beta-lactamase-encoding transposons from Staphylococcus aureus, has a novel set of putative transposition functions. Each is homologous with a well-characterized function from a different type of mobile genetic element. Thus, Tn552 encodes: (i) resL-binL, a co-integrate resolution system homologous with those of Tn3 family elements; (ii) p480, a potential transposase significantly homologous with the DNA integrases of eukaryotic retroviruses and retrotransposons; and (iii) p271, a potential ATP-binding protein that shows homology with the B protein of phage Mu. The 3' terminal nucleotides of Tn552 (CA), adjacent to which p480 might cleave, are the same as those of retroviruses, retrotransposons and phage Mu. The presumptive resolvase (BinL) is very closely related to BinR, which was identified as a DNA invertase and is now shown to resolve an artificial co-integrate in vivo. Furthermore, the structure of the derivative of Tn552 found in the staphylococcal plasmid pI258 can be explained by a BinL (or BinR)-mediated site-specific deletion ('resolution') event. Thus, pI258 contains only the right-hand half of Tn552, which encodes the beta-lactamase and two regulatory proteins. The latter are homologous with the beta-lactamase gene repressor and co-inducer of Bacillus licheniformis. Interestingly, the order of the regulatory genes is reversed in S. aureus compared with Bacillus licheniformis.