Nucleotide Sequences of Fragments of R17 Bacteriophage RNA from the Region Immediately Preceding the Coat‐Protein Cistron

The known nucleotide sequence preceding the coat protein cistron in R17 bacteriophage RNA has been extended by 23 residues towards the 5′‐terminus of the RNA. 32 P‐labelled R17 RNA was partially digested with Escherichia coli ribonuclease IV and the resulting fragments were resolved by electrophoresis on polyacrylamide gels. Fingerprinting techniques were then used to identify fragments with sequences overlapping the known sequence for the ribosomal‐binding site at the beginning of the coat protein cistron. Three such fragments (59, 31 and 26 nucleotides long) were found routinely and a slightly larger fragment was found in one experiment using a different batch of enzyme. The nucleotide sequences of the fragments were studied by established radiochemical techniques and all four fragments were found to have the same 3′‐terminus. The nucleotide sequence determined for the largest fragment is A‐A‐U‐C‐A‐G‐G‐C‐A‐A‐C‐G‐G‐C‐U‐C‐U‐C‐U‐A‐A‐A‐U‐A‐G‐A‐G‐C‐C‐C‐U‐C‐A‐A‐C‐C‐G‐G‐A‐G‐U‐U‐U‐G‐A‐A‐G‐C‐A‐U‐G‐G‐C‐U‐U‐C‐U‐A‐A‐C‐U‐U‐U where AUG is the initiation codon for the coat protein cistron. The sequence preceding this AUG contains three possible nonsense codon but whether any of them is actually used to terminate synthesis of the R17‐A protein is not known; most likely the untranslated sequence between the A protein and coat protein cistrons is at least 26 residues long. The sequence reported here shows little homology with the known sequences preceding the R17‐A protein and RNA synthetase cistrons nor with any of the sequences preceding the cistrons in Qβ phage RNA; however, it is almost identical to the sequence preceding the coat protein cistron of the closely related phage f2. The nucleotide sequence which we have determined can be folded into the form of two hydrogen‐bonded loops.