RNase HI stimulates the activity of RnlA toxin in E scherichia coli

Summary A type II toxin–antitoxin system in E scherichia coli , rnlA ‐ rnlB , functions as an anti‐phage mechanism. RnlA is a toxin with an endoribonuclease activity and the cognate RnlB inhibits RnlA toxicity in E . coli cells. After bacteriophage T 4 infection, RnlA is activated by the disappearance of RnlB , resulting in the rapid degradation of T 4 mRNAs and consequently no T 4 propagation, when T 4 dmd is defective: Dmd is an antitoxin against RnlA for promoting own propagation. Previous studies suggested that the activation of RnlA after T 4 infection was regulated by multiple components. Here, we provide the evidence that RNase HI is an essential factor for activation of RnlA . The dmd mutant phage could grow on Δ rnhA (encoding RNase HI ) cells, in which RnlA ‐mediated mRNA cleavage activity was defective. RNase HI bound to RnlA   in vivo   and enhanced the RNA cleavage activity of RnlA   in vitro . In addition, ectopic expression of RnlA in Δ rnlAB Δ rnhA cells has less effect on cell toxicity and RnlA ‐mediated mRNA degradation than in Δ rnlAB cells. This is the first example of a direct factor for activation of a toxin.