Specifically methylates the cytosines at positions 1400 (m5C1400), 1404 (m5C1404) and 1407 (m5C1407) of 16S rRNA. C1400, C1404 and C1407 are methylated in a 30S subunit substrate, but only C1400 and C1404 are methylated when naked 16S rRNA is the substrate. Methylation by RsmF may facilitate growth at temperatures outside the optimal growth temperature. cytidine(1400) in 16S rRNA + S-adenosyl-L-methionine = 5-methylcytidine(1400) in 16S rRNA + H(+) + S-adenosyl-L-homocysteine cytidine(1404) in 16S rRNA + S-adenosyl-L-methionine = 5-methylcytidine(1404) in 16S rRNA + H(+) + S-adenosyl-L-homocysteine cytidine(1407) in 16S rRNA + S-adenosyl-L-methionine = 5-methylcytidine(1407) in 16S rRNA + H(+) + S-adenosyl-L-homocysteine Belongs to the class I-like SAM-binding methyltransferase superfamily. RsmB/NOP family.