Functional interactions between chpB and parD, two homologous conditional killer systems found in the Escherichia coli chromosome and in plasmid R1.

parD and chpB are homologous conditional killer systems of plasmid and chromosomal origin, respectively. They are bicistronic operons encoding a killer protein (Kid and ChpBK) and an antidote (Kis and ChpBI). It is shown that the antidote of the chpB system can neutralize the toxin of the parD system. This activity is improved by particular amino acid changes at the amino end of the ChpBI antidote. It is further shown that the chpB system is weakly autoregulated and that the activity of a second promoter, previously identified upstream of the regulated promoter, can modulate the functional interactions between the chpB and parD systems.