Functional dichotomy in the 16s rrna (m1a1408) methyltransferase family and control of catalytic activity via a novel tryptophan mediated loop reorganization.
Methylation of the bacterial small ribosomal subunit (16S) rRNA on the N1
position of A1408 confers exceptionally high-level resistance to a broad
spectrum of aminoglycoside antibiotics. Here, we present a detailed structural
and functional analysis of the Catenulisporales acidiphilia 16S rRNA (m(1)A1408)
methyltransferase ('CacKam'). The apo CacKam structure closely resembles other
m(1)A1408 methyltransferases within its conserved SAM-binding fold but the
region linking core β strands 6 and 7 (the 'β6/7 linker') has a unique,
extended structure that partially occludes the putative 16S rRNA binding
surface, and sequesters the conserved and functionally critical W203 outside of
the CacKam active site. Substitution of conserved residues in the SAM binding
pocket reveals a functional dichotomy in the 16S rRNA (m(1)A1408)
methyltransferase family, with two apparently distinct molecular mechanisms
coupling cosubstrate/ substrate binding to catalytic activity. Our results
additionally suggest that CacKam exploits the W203-mediated remodeling of the
β6/7 linker as a novel mechanism to control 30S substrate recognition and
enzymatic turnover.
