23S ribosomal RNA is the large ribosomal RNA component of the 50S large ribosomal subunit in bacteria and archaea. It forms much of the subunit's structural framework and the RNA-based environment of the peptidyl transferase center. The functional RNA is distinct from the DNA sequence from which it is transcribed: a genomic sequence record and a mature ribosomal component describe different physical molecules.[ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. From RNA to Protein, composition and functions of ribosomal subunits
Structural experiments with an archaeal large ribosomal subunit placed two substrate analogues against conserved regions of 23S ribosomal RNA. The absence of nearby protein side chains at the bond-forming position supported the conclusion that the ribosome is a ribozyme.[ref]
The 23S label refers to a sedimentation coefficient, not 23 nucleotides. It must not be added as though it were a molecular mass: the 50S large ribosomal subunit contains 23S ribosomal RNA, 5S ribosomal RNA and ribosomal proteins, and its sedimentation depends on the assembled object's shape and resistance to motion.[ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Ribosome sizes and sedimentation labels
Thus a measurement of an isolated RNA component and a measurement of the complete subunit need not carry the same S value.