Ribonucleoproteins were obtained by T 1 ribonuclease digestion of reconstituted complexes of ribosomal protein L 1 and 23-S RNA from Escherichia coli. The RNA region of the main ribonucleoprotein 2 was totally digested with T 1 ribonuclease. The oligonucleotide products were characterised and they showed that this region comprises 148 nucleotides located between the 550th and 1000th nucleotides from the 3' end of the 23-S RNA.Of the other two ribonucleoproteins, the largest ribonucleoprotein 1 contained an extra RNA sequence, of at least 15 nucleotides, that was located at the 5' end of the RNA region. The smallest ribonucleoprotein 3 lacked an RNA section towards the 3' end of the region.The order of the RNA subfragments and the enzymic cutting positions in the whole RNA region are given for the ribonucleoproteins. It is shown that protein L 1 most strongly protects a continuous section of 115 nucleotides at the 5' end of the main RNA region.Finally, evidence is presented for a methylated base, and for two sequence heterogeneities, in this region of the 23-S RNA.It was shown in the preceding paper [l] that under suitable conditions the reconstituted complex between the ribosomal protein L1 and the 23-S RNA of Escherichia coli can be digested with T1 RNase to yield specific ribonucleoproteins. The number and the composition of these ribonucleoproteins depends upon the enzyme : RNA ratio employed in hydrolysis and upon the magnesium concentration of digestion. One of them, ribonucleoprotein 2, is obtained in a homogeneous state, and in high yield, at an intermediate enzyme : RNA ratio.In the present work, the RNA region contained in the ribonucleoprotein 2, was characterised by its total T1 RNase digestion products. This enabled us to localise the region along the sequence of the 23-S RNA molecule.Furthermore, the main RNA subfragments produced by complete denaturation of each ribonucleoprotein in urea [l] were analysed for their oligonucleo-
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