INHIBITION OF BACTERIOPHAGE MU TRANSPOSITION BY MU REPRESSOR AND FIS

In this paper we show that the Escherichia coli protein Fis has a regu latory function in Mu transposition in the presence of Mu repressor. F is can lower the transposition frequency of a mini-Mu 3-80-fold, but o nly if the Mu repressor is expressed simultaneously. In this novel typ e of regulation of transposition by the concerted action of Fis and re pressor, the IAS, the internal activating sequence, is also involved a s deletion of this site leads to the loss of the Fis effect. As the IA S contains strong repressor binding sites these are probably the targe t for the repressor in the observed negative regulation by Fis and rep ressor. However, the role of Fis and repressor is not only to inactiva te the IAS, since a 4bp insertion in the IAS, which changes the spacin g of the repressor-binding site, abolishes the enhancing function of t he IAS but leaves the repressor-Fis effect intact. A likely target for Fis in this regulation is a strong Fis-binding site, which is located adjacent to the L2 transposase-binding site. However, when this Fis-b inding sequence was substituted by a random sequence and Fis no longer showed specific binding to this site, the Fis effect was still observ ed. Although it is still possible that Fis can function by binding to this non-specific site in a particular complex, it seems more likely t hat Fis is directly or indirectly involved in determining the level of the repressor.