**4. Resistance or susceptibility to antibiotics through rRNA demethylation**

The *rsmG* gene encodes a 16S rRNA mRNA which methylates the N7 of nucleotide G527 within the 530 loop of 16S rRNA; one of the main examples is the loss of native methylations that confers a resistance phenotype to streptomycin. Streptomycin interacts with the rRNA in the adduced region (loop 530), and the loss of methylation correlates with a low level of resistance. Although this resistance is at a low dose of antibiotic, the mutation of *rsmG* apparently has a mutator effect which promotes the appearance of a high number of mutants resistant to high doses of streptomycin [36]. Another interesting aspect is that not only methylation generates resistance; cases have been reported where demethylation also promotes resistance. The first and best characterized example is *ksgA* gene (RsmA protein), which encodes the native methyltransferase KsgA or RsmA, responsible for the N6 dimethylation of A1518 and A1519 in the 3′-terminal fork of the 16S rRNA in the 30S rRNA. It was the first resistance to aminoglycosides (kasugamycin) associated with demethylation in the 16S rRNA [37]. It was found that adenine methylated by MTase is far from the binding site of kasugamycin, so this demethylation should lead to a conformational rearrangement which would be associated with the acquisition of antibiotic resistance [38].

Another research showed that preventing adenine methylation from occurring, resistance to kasugamycin can be induced; the base U793 fills the site usually occupied by the methylated adenines and the adjacent bases, A792 and A794, [39]. The phenomena mentioned above give place to a conformational change, causing the union site of Ksg to be blocked by the U793. Accordingly, it can be assured that this structural change in the helix 24 causes resistance to Ksg [39].

Likewise, it was found that the *tlyA* gene in *Mycobacterium tuberculosis* encodes the MTase 2'-O-ribose TlyA responsible for the C1409 methylations in the 16S rRNA and C1920 in the 23S rRNA. The loss of such methylations confers resistance to capreomycin and viomycin, two antibiotics which bind at the interface of the ribosome subunit and are used to help define their binding site. Another example of the absence of methylations in the 23S rRNA is the lack of methylation in U2584 (*E. coli* numbering), which causes resistance to sparsomycin in 23S rRNA *Halobacterium salinarum* [40].

Recent findings regarding intrinsic resistance refer to the Ψ at position 2504 of the 23S rRNA in *E. coli*, where inactivation of the *rluC* gene confers significant resistance to clindamycin, linezolid, and tiamulin [41]. The *cfr* gene was originally discovered in an isolate of a multiresistant plasmid during a follow-up study of chloramphenicol resistance in *Staphylococcus* spp. isolates. The molecular characterization of the resistance led to the gene encoding a methyltransferase that methylated the nucleotide A2503 in the 23S rRNA. In *E. coli* and *S. aureus*, there is a natural methylation of A2503 mediated by the methyltransferase encoded by the *yfgB* gene (*rlmN*). The lack of natural methylation in A2503 confers a slight increase in susceptibility to tiamulin, hygromycin A, sparsomycin, and linezolid [42].
