The experiments described above confirm the existence of Motif 1 and its role in reintiation in MNV. It was therefore of interest to determine the context of this 18S rRNA complementary region within the global RNA secondary structure of the minimal functional sequence, and to compare the structure with that determined for the influenza BM2 signal [19] . To achieve this, a bacteriophage T3 promoter was inserted upstream of the viral sequence of the p2luc-MNV.61 plasmid ( Figure 1b ). The plasmid was linearised with BamHI, T3 run-off transcripts synthesised and the RNA end-labelled with [ 33 P]-cATP. The labelled transcripts were subjected to limited chemical and enzymatic probing prior to analysis on denaturing polyacrylamide gels. The chemical probes used were imidazole and lead acetate, specific for cleavage of single stranded regions. Enzymatic probes were RNases T1, U2 and CL3, which preferentially cleave single-stranded G, A and C residues respectively, and RNase CV1, which cuts in helical regions in double-stranded or stacked conformations. A representative stucture mapping gel is shown in Figure 3 and in Figure 4 , the data are mapped onto mfold predictions of the secondary structure of the ''stop-start'' region.