The coat protein of bacteriophage GA, a group II RNA phage, binds to a small RNA hairpin corresponding to its replicase operator. Binding is specific, with a Ka of 71 μM-1 This interaction differs kinetically from the analogous coat protein-RNA hairpin interactions of other RNA phage and also deviates somewhat in its pH and salt dependence. Despite 46 of 129 amino acid differences between the GA and group I phage R17 coat proteins, the binding sites are fairly similar. The essential features of the GA coat protein binding site are a based-paired stem with an unpaired purine and a four nucleotide loop having an A at position -4 and a purine at -7 . Unlike the group I phage proteins, the GA coat protein does not distinguish between two alternate positions for the unpaired purine and does not show high specificity for a pyrimidine at position -5 of the loop.
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