Construction and evolution of a ribozyme in phage MS2. A. Consensus sequence and structure of the minimal hammerhead ribozyme. N, any base; H = A, C, or U; Arrow indicates site of cleavage. B. Map of the MS2 genome. The XbaI-site was used for insertion of the ribozyme sequence. C. satellite Tobacco ringspot ribozyme (regular font) lacking the native loop-loop interaction inserted in MS2 RNA (bold italics). The UAA stop codon of the extended maturation gene is boxed. D. Overview of revertants and number of occurrences. The change marked by a rectangle occurred in combination with a C3- > U mutation.