Ribosomal protein S13 is one of the proteins from the small ribosomal subunit [ 12464183]. In Escherichia coli, S13 is known to be involved in binding fMet-tRNA and, hence, in the initiation of translation. S13 contains thee helices and a beta-hairpin in the core of the protein, which form a helix-two turns-helix (H2TH) motif, and a non-globular C-terminal extension.
This H2TH motif can be found in other proteins as well. In the DNA repair protein, MutM (formamidopyrimidine DNA glycosylase; Fpg), the middle domain contains the H2TH motif. MutM is a trifunctional DNA base excision repair enzyme that removes a wide range of oxidatively damaged bases (N-glycosylase activity) and cleaves both the 3'- and 5'-phosphodiester bonds of the resulting apurinic/apyrimidinic site (AP lyase activity) [10921868]. Other repair enzymes, such as Escherichia coli Endonuclease VIII that excises oxidized pyrimidines from DNA, also contain a DNA-binding H2TH motif within the middle domain. The H2TH domains of these repair proteins are only peripherally involved in binding DNA; their primary function may be simply to position the N-terminal lobe and C-terminal zinc finger domain of the glycosylases for interactions with DNA.
The middle domain of topoisomerase IV-B subunit contains a H2TH motif that is structurally related to the DNA repair proteins. Although the H2TH domain appears to be retained in all archaeal and plant typeż IIB topoisomerases identified to date, it has no known function and has not been observed in other topoisomerase families [12505993]. | 
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