Specific recognition of the isolated R17 replicase initiator region by R17 coat protein

Abstract

COAT proteins from the RNA bacteriophages of groups I (R17, MS2,f2) and III (Qβ) specifically bind to their homologous RNAs and serve as translational repressers of replicase synthesis¹. In the case of R17, a fragment of the genome which is protected by coat protein from T₁ RNase digestion² contains not only the ribosome binding site of the replicase cistron³, but also the preceding intercistronic region and 23 nucleotides (including the terminator triplets) from the coat gene⁴. Physical studies described in the previous papers^5,6 suggest that the structure of this isolated R17 fragment consists of two hairpin loops joined by a single-stranded region; the coat protein binds in such a way as to alter the behaviour of only one of these helical regions⁵, that which contains the initiation site for the replicase gene. Here I show that this portion of the fragment alone is sufficient for recognition by the coat protein.