Toxic component of a type II toxin-antitoxin (TA) system. Has RNase activity (By similarity). Is very toxic upon expression in E.coli. Its toxic activity is probably neutralized by the cognate antitoxin RelB3. Forms heterodimers with RelB3 and possibly a heterotetramer RelE3-RelB3(2)-RelE3 from 2 heterodimers. The heterotetramer is probably not very stable in solution. Belongs to the RelE toxin family.