In оne fоrm оf trаnscriptionаl terminаtion in bacteria, a helicase protein binds to a sequence in the mRNA and moves along it toward RNA polymerase, while the polymerase transcribes a sequence which immediately forms a stem-loop. The stem-loop binds to and stalls the polymerase. The helicase catches up to the polymerase and separates the DNA-RNA hybrid. This is called _____.