Figure 1. Secondary structures of representative RPRs from the three domains of life. (a) Bacteria (type A, E. coli); (b) Bacteria (type B, B. subtilis); (c) Archaea (P. furiosus); and (d) Eukarya (H. sapiens). Helices (or paired regions) are labeled as P1, P2, etc. consecutively from 5′ to 3′; a linker joining two helices P11 and P12 is termed J11/12, and a loop capping a specific helix such as P3 termed L3. Universally conserved nucleotides in RPRs from all three domains are depicted by arrowheads. CR refers to conserved regions; CR III, albeit bereft of universally conserved nucleotides, does exhibit high conservation of identity. The catalytic (C) and specificity (S) domains in bacterial RPR are indicated in black and blue color, respectively. The tertiary contacts that are unique to bacterial RPR are highlighted by red lines. This figure is modified from versions available in the RNase P database (Brown 1999).