This is the catalytic core of this SAM-dependent 16S ribosomal methyltransferase RsmB/F enzyme [1,2]. There is a catalytic cysteine residue at 180 in UniProtKB:Q5SII2 with another highly conserved cysteine at residue 230. It methylates the C(5) position of cytosine 2870 (m5C2870) in 25S rRNA [1,2].
The C-terminal catalytic domain of ribosomal RNA cysteine methyltransferases is highly conserved in archaeal, bacterial and eukaryotic proteins [PUBMED:21123870], such as ribosomal RNA methyltransferase B (RsmB, Sun, Fmu) and Nop2. Escherichia coli RsmB methylates cysteine C967 in 16S rRNA [PUBMED:21123870]. Nop2 methylates cysteine C2870 in the 25S rRNA of S. cerevisiae [PUBMED:23913415] and is critical for 60S biogenesis [PUBMED:8972218].
Ishikawa I, Sakai N, Tamura T, Yao M, Watanabe N, Tanaka I;, Proteins. 2004;54:814-816.: Crystal structure of human p120 homologue protein PH1374 from Pyrococcus horikoshii. PUBMED:14997580 EPMC:14997580 .
Demirci H, Larsen LH, Hansen T, Rasmussen A, Cadambi A, Gregory ST, Kirpekar F, Jogl G;, RNA. 2010;16:1584-1596.: Multi-site-specific 16S rRNA methyltransferase RsmF from Thermus thermophilus. PUBMED:20558545 EPMC:20558545.