The rapidly labeled nuclear ribonucleic acid in human carcinoma cells which is protected by protein from digestion by staphylococcal nuclease (EC 126.96.36.199) has been investigated. A simple and discrete sequence specificity was not found, but the protected RNA fragments are rich in G + C and were shown by fingerprinting to comprise a non-random subset of all heterogeneous nuclear ribonucleic acid (hnRNA) sequences enriched in the sequences AGC, GGC, AGGC, and GAGC. There was no detectable enrichment for double-stranded RNA in the protected fraction. These data provide the first evidence that the association of any protein with hnRNA is nonrandom with respect to nucleotide sequence.
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