Antitoxin component of a type II toxin-antitoxin (TA) system. A labile antitoxin (half-life of 2.1 minutes) that inhibits the endonuclease activity of cognate toxin RnlA but not that of non-cognate toxin LsoA. Can form a complex with cognate toxin RnlA. Not repressed by IscR. rnlA-rnlB forms an operon, the downstream rnlB also has its own promoter. Probably degraded by CplXP and Lon proteases. Essential, it cannot be deleted unless rnlA is also disrupted. Truncated N-terminus.