S-adenosyl-L-methionine-dependent methyltransferase that specifically methylates the C(5) position of cytosine 3438 (m5C3438) in 28S rRNA (PubMed:31722427). m5C3782 promotes protein translation without affecting ribosome biogenesis and fidelity (By similarity). Required for corpus callosum and cerebral cortex development (PubMed:31174389, PubMed:31462248). a cytidine in 28S rRNA + S-adenosyl-L-methionine = a 5-methylcytidine in 28S rRNA + H(+) + S-adenosyl-L-homocysteine In the hippocampus, specifically expressed in adult hippocampal NG2-positive oligodendrocyte precursor cells (at protein level). Present in the developing cerebral cortex from embryonic day 12.5 dpc, with a peak at 14.5 dpc followed by a decrease from 18.5 dpc (at protein level) (PubMed:31462248). Selectively expressed in radial glial cells of cerebral cortex from 12.5 to 16.5 dpc, but not in intermediate progenitor cells (IPCs) or neocortical neurons (at protein level) (PubMed:31462248). Highly expressed in callosal oligodendrocyte precursor cells (OPCs) and oligodendrocytes (OLs) from postnatal day 7 to postnatal day 28 (PubMed:31174389). Decreased body weight and lean mass without alterations in food intake (PubMed:31722427). Adult mice show spatial cognitive deficits, possibly caused by defects in development and function of oligodendrocyte precursor cells (PubMed:30485550). Mice display a reduction of the corpus callosum with a decline in the number of myelinated axons and loose myelin sheath (PubMed:31174389). They also show impaired development of the cerebral cortex, characterized by impaired growth of radial glial scaffold (PubMed:31462248). Belongs to the class I-like SAM-binding methyltransferase superfamily. RsmB/NOP family.