FUNCTIONAL INTERACTIONS BETWEEN HOMOLOGOUS CONDITIONAL KILLER SYSTEMSOF PLASMID AND CHROMOSOMAL ORIGIN

parD and chpA are homologous conditional killer systems of plasmid and chromosomal origin, respectively, encoding a killer protein (Kid and ChpAK) and an antidote (Kis and ChpAI). Here it is shown that these sy stems can functionally interact. A multicopy chpA recombinant partiall y complements two mutations in the antidote of the parD system. These mutations affect either autoregulation or neutralization of the killer component. Following in vitro mutagenesis with hydroxylamine, chpA mu tants that improve this complementation were isolated. Sequence analys is shows that these mutants are clustered in the 5' end of the chpAI g ene and structure predictions suggest that they affect a putative loop in the secondary structure of the ChpAI antidote. It is proposed that this region is part of a protein-protein interface required for the f unctional interaction between the antidote and the killer components i n the two homologous systems.