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].

1: Crystal structure of human p120 homologue protein PH1374 from Pyrococcus horikoshii. Ishikawa I, Sakai N, Tamura T, Yao M, Watanabe N, Tanaka I; Proteins. 2004;54:814-816. PMID:14997580

2: Multi-site-specific 16S rRNA methyltransferase RsmF from Thermus thermophilus. Demirci H, Larsen LH, Hansen T, Rasmussen A, Cadambi A, Gregory ST, Kirpekar F, Jogl G; RNA. 2010;16:1584-1596. PMID:20558545