Characterization of a functional insertion sequence IS Sau2 from Staphylococcus aureus .

Background: IS Sau2 has been suggested as a member of the IS 150 f subgroup in the IS3 family. It encodes a fusion transposase OrfAB produced by programmed - 1 translational frameshifting with two overlapping reading frames orfA and orfB . To better characterize IS Sau2 , the binding and cleaving activities of the IS Sau2 transposase and its transposition frequency were studied.

Results: The purified IS Sau2 transposase OrfAB was a functional protein in vitro since it bound specifically to IS Sau2 terminal inverted repeat sequences (IRs) and cleaved the transposon ends at the artificial mini-transposon pUC19-IRL-gfp-IRR. In addition, the transposition frequency of IS Sau2 in vivo was approximately 1.76 ± 0.13 × 10- 3 , based on a GFP hop-on assay. Furthermore, OrfB cleaved IRs with the similar catalytic activity of OrfAB, while OrfA had no catalytic activity. Finally, either OrfA or OrfB significantly reduced the transposition of IS Sau2 induced by OrfAB.

Conclusion: We have confirmed that IS Sau2 is a member of IS 150 /IS 3 family. The IS Sau2 transposase OrfAB could bind to and cleave the specific fragments containing the terminal inverted repeat sequences and induce the transposition, suggesting that IS Sau2 is at least partially functional. Meanwhile, both OrfA and OrfB inhibited the transposition by IS Sau2 . Our results will help understand biological roles of IS Sau2 in its host S. aureus .