gidB

gidB is a gene found in many bacteria encoding a small, cytosolic S-adenosyl-L-methionine (SAM)-dependent methyltransferase of the GidB family. It is conserved across diverse bacterial lineages. The encoded protein methylates guanine at position 527 of 16S ribosomal RNA (G527) to form m7G527, a modification located within the decoding center of the 30S subunit near the streptomycin binding site.

Functionally, the GidB-catalyzed modification contributes to proper ribosome function and translational fidelity. The presence or absence of the methyl group alters the local rRNA structure and can modulate binding of aminoglycoside antibiotics such as streptomycin.

Clinical relevance: Mutations in gidB, or loss of gidB function, are associated with low-level resistance to streptomycin in several bacteria, most notably in Mycobacterium tuberculosis. In TB, gidB alterations are common in strains that harbor streptomycin resistance and can be found alongside other resistance determinants, complicating molecular diagnostics that target rpsL or rrs.

Genetics and structure: gidB is typically nonessential in many bacteria and exists as a single-copy gene. GidB proteins feature the characteristic Rossmann-like SAM-dependent methyltransferase fold and use SAM as methyl donor to modify G527 of 16S rRNA.

Distribution: gidB homologs are broadly distributed in bacteria, reflecting the conserved nature of rRNA modification. The specific phenotypic effect of gidB mutations may vary by species.