EVOLUTIONARY VARIATION IN BACTERIAL RNASE-P-RNAS

Sequences encoding RNase P RNAs from representatives of the last remai ning classical phyla of Bacteria have been determined, completing a ge neral phylogenetic survey of RNase P RNA sequence and structure. This broad sampling of RNase P RNAs allows some refinement of the secondary structure, and reveals patterns in the evolutionary variation of sequ ences and secondary structures, Although the sequences range from 100 to <25% identical to one another, and although only 40 of the nucleoti des are invariant, there is considerable conservation of the underlyin g core of the RNA sequence. RNase P RNAs, like group I intron RNAs but unlike ribosomal RNAs, transfer RNAs or other highly conserved RNAs, are quite variable in secondary structure outside of this conserved st ructural core. Conservative regions of the RNA evolve by substitution of apparently interchangeable alternative structures, rather than the insertion and deletion of helical elements that occurs in the more var iable regions of the RNA. In a remarkable case of convergent molecular evolution, most of the unusual structural elements of type B RNase P RNAs of the low G+C Gram-positive Bacteria have evolved independently in Thermomicrobium roseum, a member of the green non-sulfur Bacteria.