<p> The S1 domain of around 70 amino acids, originally identified in ribosomal protein S1, is found in a large number of RNA-associated proteins. It has been shown that S1 proteins bind RNA through their S1 domains with some degree of sequence specificity. This type of S1 domain is found in translation initiation factor 1.</p><p> The solution structure of one S1 RNA-binding domain from <taxon tax_id="562">Escherichia coli</taxon> polynucleotide phosphorylase has been determined [<cite idref="PUB00000944"/>]. It displays some similarity with the cold shock domain (CSD) (<db_xref db="INTERPRO" dbkey="IPR002059"/>). Both the S1 and the CSD domain consist of an antiparallel beta barrel of the same topology with 5 beta strands. This fold is also shared by many other proteins of unrelated function and is known as the OB fold. However, the S1 and CSD fold can be distinguished from the other OB folds by the presence of a short 3(10) helix at the end of strand 3. This unique feature is likely to form a part of the DNA/RNA-binding site.</p><p> This entry is specific for bacterial, chloroplastic and eukaryotic IF-1 type S1 domains.</p>