The proposed minimal PTC. The upper view is from above the two 23S helices that form the "base" of the PTC. The RNA segment from 2472–2650 (1S72.pdb) is shown as cyan ribbon strands representing the self-folding minimal PTC segment. The red is the adjoining small self-folding RNA additional helix 2427 to 2462, potentially displaced by L10e/L16 (see text). There are five proteins represented both in space fill and gray backbone. Their extensions are L22 (115–143) in green; L15 (1–57) in red; L6 (139–172) in orange; L4 (42–100) in yellow; L3 (1–21 and 205–262) in blue. L10e is colored cyan and is fully displayed in space fill to emphasize its mimic of a third RNA helix, forming part of the PTC base. The arrow shows the direction and location of the peptide exit tunnel. The lower figure is an on-edge view of the plane formed by the minimal PTC 23S RNA self-folding segment and the contacting peptide extensions. The black line shows the position of the plane formed by the extant PTC base RNA helices and L10e. This line also suggests a potential membrane surface upon which an early PTC function may have evolved (see Discussion section).