It is well established that when E. coil 30s ribosomal subunits are irradiated with ultraviolet light under mild conditions a specific cross-link is formed between protein S7 and the 16s RNA. Methodology is presented for the analysis of the single nucleotide residue concerned in this cross-link. Firstly, the identity of the ribonuclease T1 octanucleotide attached to S7 is confirmed by a new method, which involves isolation and analysis of S7-polynucleotide complexes containing 30 - 40 nucleotides. Secondly, the isolated S7-octanucleotide complex is digested successively with ribonuclease A, proteinase K and ribonuclease T2, and the nucleotides liberated are identified. The results show unambiguously that uridine residue number 1239 in the 16s RNA sequence is cross-linked to protein S7.
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