This family of proteins include rRNA adenine dimethylases (e.g. KsgA) and the Erythromycin resistance methylases (Erm).

The bacterial enzyme KsgA catalyzes the transfer of a total of four methyl groups from S-adenosyl-l-methionine (S-AdoMet) to two adjacent adenosine bases in 16S rRNA. This enzyme and the resulting modified adenosine bases appear to be conserved in all species of eubacteria, eukaryotes, and archaea, and in eukaryotic organelles. Bacterial resistance to the aminoglycoside antibiotic kasugamycin involves inactivation of KsgA and resulting loss of the dimethylations, with modest consequences to the overall fitness of the organism. In contrast, the yeast ortholog, Dim1, is essential. In yeast, and presumably in other eukaryotes, the enzyme performs a vital role in pre-rRNA processing in addition to its methylating activity. The best conserved region in these enzymes is located in the N-terminal section and corresponds to a region that is probably involved in S-adenosyl methionine (SAM) binding domain.

The crystal structure of KsgA from Escherichia coli has been solved to a resolution of 2.1A. It bears a strong similarity to the crystal structure of ErmC' from Bacillus stearothermophilus and a lesser similarity to the yeast mitochondrial transcription factor, sc-mtTFB [PMID:15136037].

The Erm family of RNA methyltransferases, which methylate a single adenosine base in 23S rRNA confer resistance to the MLS-B group of antibiotics. Despite their sequence similarity, the two enzyme families have strikingly different levels of regulation that remain to be elucidated. Other orthologs, of this family include the yeast (MTF1, [PMID:11567089]) and human (h-mtTFB) mitochondrial transcription factors, which are nuclear encoded. Human-mtTFB is able to stimulate transcription in vitro independently of its S-adenosylmethionine binding and rRNA methyltransferase activity [PMID:12897151].