The Peptidyltransferase (PT) region (bases 2469-2589) of the E. coli 23S rRNA was found to be significantly homologous to E. coli SS rRNA. These two sequences have 54.4% identity; the corresponding matching probability by chance (P(nuc)) is 0.21 x 10(-10). This finding suggests that primitive rRNA co-ancestral to SS rRNA and the PT region of 23S rRNA might have engaged in the primitive PT reaction in early polypeptide synthesis. Evolutionary significance of this finding is discussed in relation to the origin of the genetic apparatus, and to the previously proposed "small ribosome theory" and "primordial gene theory".
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