F plasmid CcdB killer protein ccdB gene mutants coding for non‐cytotoxic proteins which retain their regulatory functions

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Abstract
The ccd locus of the F plasmid codes for two gene products, CcdA and CcdB, which contribute to the plasmid's high stability by post-segregational killing of plasmid-free bacteria. Like the quinolones, the CcdB protein is a poison of the DNA-topoisomerase II complexes, while CcdA acts as an antidote against CcdB. in addition to these poison-antipoison properties, the CcdA and CcdB proteins act together at transcription level to repress their own synthesis. In this work, we have isolated, in vivo., and characterized several non-killer CcdB mutants. All missense mutations which inactivate CcdB killer activity are located in the region coding for the last three C-terminal residues. However, the resulting mutant CcdB proteins retain their auto-regulatory properties. We conclude that the last three C-terminal residues of CcdB play a key role in poisoning but are not involved in repressor formation.