InterPro domain: IPR006463

This entry represents the MiaB enzyme and homologues that are responsible for the modification of the isopentenylated adenine-37 base of most bacterial and eukaryotic tRNAs that read codons beginning with uracil (all except tRNA(I,V) Ser). Adenine-37 is next to the anticodon on the 3' side in these tRNA's, and lack of modification at this site leads to an increased spontaneous mutation frequency. Isopentenylated A-37 is modified by methylthiolation at position 2 leading to the formation of 2-methylthio-N6-(dimethylallyl)adenosine, either by MiaB alone or in concert with a separate methylase yet to be discovered [ 1 ]. MiaB contains two 4Fe-4S clusters which are labile under oxidizing conditions [ 2 ,[ 3 ]. One cluster is coordinated with three cysteines and an exchangeable S-adenosyl-L-methionine, the other is thought to be the sulfur donor. The second cluster has a polysulfide group bound to it, which is methylated in the first reaction step. Concurrently, the SAM-[4Fe-4S] cluster forms the 5'-dAdo radical, which abstracts a hydrogen atom from the substrate, which is then methylthiolated by the methylated polysulfide group [ 4 , 5 , 6 ]. Additionally, the sequence is homologous to the biotin synthetase, BioB, which utilises both an iron-sulfur cluster and S-adenosym methionine (SAM) to generate a radical which is responsible for initiating the insertion of sulfur into the substrate [ 7 ]. It is reasonable to surmise that the methyl group of SAM becomes the methyl group of the product, but this has not been shown, and the possibility of a separate methylase exists. These enzymes contain a TRAM domain [ 8 ] which is believed to be responsible for binding to tRNAs. The enzyme from Thermotoga maritima has been cloned, expressed, spectroscopically characterised and shown to complement the E. coli MiaB enzyme [ 8 ].

1. Identification of the miaB gene, involved in methylthiolation of isopentenylated A37 derivatives in the tRNA of Salmonella typhimurium and Escherichia coli. J. Bacteriol. 181, 7256-65
2. Enzymatic modification of tRNAs: MiaB is an iron-sulfur protein. J. Biol. Chem. 277, 13367-70
3. MiaB, a bifunctional radical-S-adenosylmethionine enzyme involved in the thiolation and methylation of tRNA, contains two essential [4Fe-4S] clusters. Biochemistry 46, 5140-7
4. MiaB protein from Thermotoga maritima. Characterization of an extremely thermophilic tRNA-methylthiotransferase. J. Biol. Chem. 278, 29515-24
5. MiaB protein is a bifunctional radical-S-adenosylmethionine enzyme involved in thiolation and methylation of tRNA. J. Biol. Chem. 279, 47555-63
6. Identification of an intermediate methyl carrier in the radical S-adenosylmethionine methylthiotransferases RimO and MiaB. J. Am. Chem. Soc. 135, 15404-16
7. TRAM, a predicted RNA-binding domain, common to tRNA uracil methylation and adenine thiolation enzymes. FEMS Microbiol. Lett. 197, 215-21