Mapping evolution with ribosome structure: intralineage constancy and interlineage variation.

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Abstract
Ribosomal small subunits are organized in 3 general structural patterns that correspond to the eubacterial, archaebacterial and eukaryotic lineages. Within each of these lineages, ribosomal structure is highly conserved. Small subunits from all 3 lineages share a common overall structure except for the following differences: small subunits from archaebacteria and from the cytoplasmic component of eukaryotes both contain a feature on the head of the subunit, the archaebacterial bill, that is absent in eubacteria; and eukaryotic small subunits contain additional regions of density at the base of the subunit, the eukaryotic lobes, that are absent in archaebacteria and in eubacteria. The intralineage conservation of ribosomal 3-dimensional structure forms a phylogenetic basis for regarding archaebacteria, eubacteria and eukaryotes as primitive lines. Although these data are separate and independent from those of Woese and Fox, they lend further support to their proposal. These data also provide a simple, rapid and accurate method for classifying organisms and for identifying new lineages. Interlineage variation of ribosomal structure is used to establish a rigorous framework for considering the evolution of these 3 lines.