Secondary structure model for the first three domains of Q beta RNA. Control of A-protein synthesis.

We present a secondary structure model for the first 860 nucleotides of Q beta RNA. The model is supported by phylogenetic comparison, nuclease S1 structure probing and computer prediction using energy minimization and a Monte Carlo approach. To provide the necessary data for the comparative analysis we have sequenced the single-stranded RNA coliphages MX1, M11 and NL95. Together with the known sequences of Q beta and SP, this yields five sequences with sufficient sequence diversity to be useful for the analysis. The part of the Q beta genome examined contains the 60 nucleotide 5' untranslated region and the first 800 nucleotide of the maturation protein gene. The RNA adopts a highly ordered structure in which all hairpins are held in place by a network of long-distance interactions, which form three-way and four-way junctions. Only the 5'-terminal hairpin is unrestrained, while connected by a few single-stranded nucleotides to the body of the RNA. The start region of the A-protein gene, which is part of the network of long-distance interactions, is base-paired to three non-contiguous downstream sequences. As a result, translation is expected to be progressively quenched when the length of the nascent chains increases. This feature explains the previous observation that A-protein synthesis on Q beta RNA can start only on short nascent strands. Translational control of the A protein in the distantly related phage MS2 was recently shown to be controlled by the kinetics of RNA folding. This basic difference and its possible biological purpose can be explained by the different RNA folding pathways in Q beta and MS2. Interestingly, due to the presence of G-U pairs, structure prediction for the minus strand differs in some aspects from that for the plus strand. More specifically, there is a minus-strand specific, long-distance interaction bordering the minus-strand equivalent of the 5'-terminal hairpin. This interaction extends at the expense of the lower part of the terminal helix, thereby exposing the terminal C residues at which replication starts. This long-distance interaction, which was recently shown to be required for minus-strand replication, is strongly supported by our comparative data.