THE TN5 TRANSPOSON

The bacterial transposon Tn5 encodes two proteins, the transposase and a related protein, the transposition inhibitor, whose relative abunda nce determines, in part, the frequency of Tn5 transposition. The synth esis of these proteins is programmed by a complex set of genetic regul atory elements. The host DNA methylation function, dam, inhibits trans posase 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 f or the transposase's recognition of the 19-bp end DNA sequences. This is the first critical step in the transposition process. Transposase-e nd DNA interaction is itself regulated by an intricate series of react ions involving several host proteins: DnaA, Dam, and Fis. The transpos ase is a unique protein in that it acts primarily in cis and inhibits its own activity in trans. Models to explain these properties are desc ribed. Finally circumstantial evidence suggests that transposition occ urs preferentially from newly replicated DNA that has yet to be partit ioned to progeny cells. This timing of transposition is likely to have a selective advantage for the host and the transposable element.