Toxic component of a type IV toxin-antitoxin (TA) system (PubMed:14594833, PubMed:28257056, PubMed:28931012). Acts as a toxin inhibitor that blocks cell division and cell elongation via FtsZ and possibly also MreB (although no interaction with MreB has been proven) (PubMed:28931012). Overexpression results in inhibition of growth in liquid cultures and a decrease in colony formation (PubMed:14594833, PubMed:28257056, PubMed:28931012). These effects are overcome by concomitant expression of cognate antitoxin YafW, which leads to toxin loss by an unknown mechanism (PubMed:14594833, PubMed:28257056). Overexpression leads to formation of lemon-shaped cells and cell lysis; inactivated by overexpression of cognate antitoxin YafW but not when the 2 genes are coexpressed from the same plasmid (PubMed:28257056). Also neutralized by overexpression of non-cognate antitoxins YfjZ and CbeA (PubMed:28257056). Co-overexpression of toxin YkfI and antitoxin YafW leads to formation of elongated cells (PubMed:28257056). Interacts with FtsZ but not MreB. Expressed in mid-log phase at lower levels than toxin relE. Deletion of 3 type IV toxin genes (cbtA, ykfI, ypfJ) leads to a slight reduction in resistance to oxidative stress, has no effect on cell growth. Encoded in prophage CP4-6. Belongs to the CbtA/YkfI/YpjF toxin family.