Conformation of Methylated Sequences in HeLa Cell 18‐S Ribosomal RNA: Nuclease S1 as a Probe

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Abstract
18-S rRNA from HeLa [human cervical cancer] cells was digested with nuclease S1 [EC 3.1.4.21]. Under the conditions employed 15% of the total nucleotides and some 50% of the methylated nucleotides were released as low molecular weight products. The material which was precipitable by 70% ethanol after nuclease S1 digestion was subjected to further digestion by combined T1 plus pancreatic RNases or by T1 RNase alone, and fingerprints were prepared. The 4 sites which are modified late during ribosome maturation, and which contain base modifications, were accessible to nuclease S1. By contrast fewer than 1/2 of the sites which are modified early during ribosome maturation, and which contain 2''-O-methyl groups, were accessible to nuclease S1. The remainder were protected, presumably by secondary or tertiary interactions within 18-S rRNA.