THE TN5 TRANSPOSON

The bacterial transposon Tn5 encodes two proteins, the transposase and a related protein, the transposition inhibitor, whose relative abundance determines, in part, the frequency of Tn5 transposition. The synthesis of these proteins is programmed by a complex set of genetic regulatory elements. The host DNA methylation function, dam, inhibits transposase promoter recognition and indirectly enhances the transposition inhibitor promoter. The inhibitor lacks the N-terminal 55 amino acids of the transposase, suggesting that this sequence plays a key role in the transposition process. An intact N-terminal sequence is required for the transposase's recognition of the 19-bp end DNA sequences. This is the first critical step in the transposition process. Transposase-end DNA interaction is itself regulated by an intricate series of reactions involving several host proteins: DnaA, Dam, and Fis. The transposase is a unique protein in that it acts primarily in cis and inhibits its own activity in trans. Models to explain these properties are described. Finally circumstantial evidence suggests that transposition occurs preferentially from newly replicated DNA that has yet to be partitioned to progeny cells. This timing of transposition is likely to have a selective advantage for the host and the transposable element.