The amino acid sequence and crystal structure of the ribosomal protein S6 from the small ribosomal subunit of Thermus thermophilus have been determined. S6 is a small protein with 101 amino acid residues. The 3D structure, which was determined to 2.0 A resolution, consists of a four-stranded anti-parallel beta-sheet with two alpha-helices packed on one side. Similar folding patterns have been observed for other ribosomal proteins and may suggest an original RNA-interacting motif. Related topologies are also found in several other nucleic acid-interacting proteins and based on the assumption that the structure of the ribosome was established early in the molecular evolution, the possibility that an ancestral RNA-interacting motif in ribosomal proteins is the evolutionary origin for the nucleic acid-interacting domain in large classes of ribonucleic acid binding proteins should be considered.