Comparison of RNA secondary structures in the 3' UTR in four representative RNA phages. SP is a species IV allolevivirus. PP7 and AP205 resemble SP in their simple 3' UTR structure. MS2 has a more elaborate folding with many extra elements (shaded). The stopcodon of replicase is boxed (R stop). Also boxed is a conserved sequence in the top of U1 (UGCUU). In Q(beta) and SP these nucleotides form a pseudoknot with their complement 1200 nts upstream (Fig. 15-6). For MS2, PP7 and AP205 a similar interaction is supposed to exist.