Branch Migration ComplexUniversity of East Anglia, Norwich, U.K.Colicins are protein toxins secreted by bacteria during times of nutrient or environmental stress as a means of reducing competition from other microbial populations. These mulitdomain toxins parasitize existing nutrient import pathways, such as those responsible for iron and vitamin uptake, analogous to bacteriophage infection. Through these routes they are able to deliver cytotoxic domains such as ionophores into the periplasmic space. Our work has focused on endonuclease colicins which have to traverse both membranes of an E.coZi cell in order to deliver a 15 kDa, non-specific DNase domain to the cytoplasm. DNase colicin-producing bacteria avoid suicide by co-expressing a small acidic immunity protein which binds to the DNase and neuhalizes its activity. The presentation will focus on the mode of action of the immunity protein Im9. lm9 folds into its distorted antiparallel 4-helix bundlestructure in