Localisation of major Q(beta) polymerase (R) and host factor (H) binding sites in the M site region (RD1) of Q(beta) RNA. The structure shown starts at nt 2421 and runs to the very 3' end (bold). For clarity several subdomains and RD2 are presented only as dashed circles. The region screened for protection by R and H extends from nt 2550 to 3100. The folding model is based on structure probing, on phylogenetic comparison with MX1, M11, SP (33) and NL95, and on computer prediction. Taken from Beekwilder (6), Adhin et al. (2) and Klovins and van Duin (42) RD1; replicase domain 1. RD2; replicases domain 2. 3' TD; 3' terminal domain. The replicase stop codon at nt 4119 is in bold italics (UAA).