Interplay of the bacterial ribosomal A-site, S12 protein mutations and paromomycin binding: a molecular dynamics study.

The conformational properties of the aminoacyl-tRNA binding site (A-site), and its surroundings in the Escherichia coli 30S ribosomal subunit, are of great relevance in designing antibacterial agents. The 30S subunit A-site is near ribosomal protein S12, which neighbors helices h27 and H69; this latter helix, of the 50S subunit, is a functionally important component of an intersubunit bridge. Experimental work has shown that specific point mutations in S12 (K42A, R53A) yield hyper-accurate ribosomes, which in turn confers resistance to the antibiotic 'paromomycin' (even when this aminoglycoside is bound to the A-site). Suspecting that these effects can be elucidated in terms of the local atomic interactions and detailed dynamics of this region of the bacterial ribosome, we have used molecular dynamics simulations to explore the motion of a fragment of the E. coli ribosome, including the A-site. We found that the ribosomal regions surrounding the A-site modify the conformational space of the flexible A-site adenines 1492/93. Specifically, we found that A-site mobility is affected by stacking interactions between adenines A1493 and A1913, and by contacts between A1492 and a flexible side-chain (K43) from the S12 protein. In addition, our simulations reveal possible indirect pathways by which the R53A and K42A mutations in S12 are coupled to the dynamical properties of the A-site. Our work extends what is known about the atomistic dynamics of the A-site, and suggests possible links between the biological effects of hyper-accurate mutations in the S12 protein and conformational properties of the ribosome; the implications for S12 dynamics help elucidate how the miscoding effects of paromomycin may be evaded in antibiotic-resistant mutants of the bacterial ribosome.