This chapter focuses on the crystal structure of the RNA binding domain of BstL2, and discusses its structure from functional and evolutionary points of view. Site-directed mutagenesis of Arg86 or Arg155 significantly diminished RNA binding affinity, and in addition, Arg68 and Lys70 mutations caused partial loss of RNA binding. To date, the three-dimensional structures of over a dozen ribosomal proteins have been determined. Comparison of their structures with those of other known proteins in the Protein Data Bank revealed that many ribosomal proteins share structural motifs, such as RNP, dsRNA binding domain, KH domain, and helix-turn-helix motifs, with RNA or DNA binding proteins. Recent studies of Thermus aquaticus ribosomes, however, demonstrated that peptidyltransferase activity is never attributed solely to the 23S rRNA, and they reduce the number of possible essential macromolecular components of the peptidyltransferase center to 23S rRNA and ribosomal proteins L2 and L3.