A Mechanism for Tn5Inhibition

Abstract

Tn5 is unique among prokaryotic transposable ele-ments in that it encodes a special inhibitor protein iden-tical to the Tn5 transposase except lacking a short NH 2 -terminal DNA binding sequence. This protein regulates transposition through nonproductive protein-protein interactions with transposase. We have studied the mechanism of Tn5 inhibition in vitro and find that a heterodimeric complex between the inhibitor and trans-posase is critical for inhibition, probably via a DNA-bound form of transposase. Two dimerization domains are known in the inhibitor/transposase shared se-quence, and we show that the COOH-terminal domain is necessary for inhibition, correlating with the ability of the inhibitor protein to homodimerize via this domain. This regulatory complex may provide clues to the struc-tures of functional synaptic complexes. Additionally, we find that NH 2 -and COOH-terminal regions of trans-posase or inhibitor are in functional contact. The NH 2 terminus appears to occlude transposase homodimer-ization (hypothetically mediated by the COOH termi-nus), an effect that might contribute to productive transposition. Conversely, a deletion of the COOH ter-minus uncovers a secondary DNA binding region in the inhibitor protein which is probably located near the NH 2 terminus.