Acceptance or rejection of various RNA structure elements introduced in a non-coding region of MS2. At the left we show the wild-type structure, containing the stopcodon (UAG italics) of the maturation gene. Mutant A has been given a big unstructured loop that is almost fully deleted during passaging (double arrows). Mutant C, containing a nearly perfect hairpin extension, is genetically stable for many generations. Mutant R with the perfect hairpin extension is an RNase III substrate. In RNase III-deficient strains the stem is genetically stable but in wild-type E. coli suppressor mutations occur that make the stem RNase III resistant (bold italics top right). We show only some representative examples of the pseudorevertants obtained.