Structural insights into +1 frameshifting promoted by expanded or modification-deficient anticodon stem loops

Abstract

Maintenance of the correct reading frame on the ribosome is essential for accurate protein synthesis. Here, we report structures of the 70S ribosome bound to frameshift suppressor tRNA SufA6 and N1-methylguanosine at position 37 (m 1 G37) modification-deficient anticodon stem loop Pro , both of which cause the ribosome to decode 4 rather than 3 nucleotides, resulting in a +1 reading frame. Our results reveal that decoding at +1 suppressible codons causes suppressor tRNA SufA6 to undergo a rearrangement of its 5′ stem that destabilizes U32, thereby disrupting the conserved U32-A38 base pair. Unexpectedly, the removal of the m 1 G37 modification of tRNA Pro also disrupts U32-A38 pairing in a structurally analogous manner. The lack of U32-A38 pairing provides a structural correlation between the transition from canonical translation and a +1 reading of the mRNA. Our structures clarify the molecular mechanism behind suppressor tRNA-induced +1 frameshifting and advance our understanding of the role played by the ribosome in maintaining the correct translational reading frame.near cognate | processivity T he three polymerase reactions of DNA replication, RNA transcription, and protein translation are essential to life and all involve a delicate balance between speed and fidelity. The bacterial ribosome decodes three mRNA nucleotides into a single amino acid at a rate of ∼20 residues/s with high fidelity (10 3

Structural insights into translational recoding by frameshift suppressor tRNASufJ

“…The structures here reveal that ASL SufJ binds to three +1 suppressible codons in the zero frame allowing only a three-nucleotide codon-anticodon cognate interaction in the A site. The C31.5 insertion does not appear to alter the structural integrity of the ASL in the same manner as other frameshift suppressors where disordering of the conserved U turn, the entire 5 ′ stem or nucleotide 32 occurs (Phelps et al 2006;Dunham et al 2007;Maehigashi et al 2014). Instead, ASL SufJ accommodates the insertion through concerted movements of both the 5 ′ and 3 ′ phosphate backbones of the anticodon stem, causing a reorganization of key nucleotide interactions in the anticodon loop.…”

TrmD

“…Importantly, the resulting m 1 G37 has the ability to remodel the structure of the tRNA anticodon loop relative to the unmethylated G37. In X-ray crystal structures of the ASL domain of tRNA Pro/CGG (CGG: anticodon) in complex with a Thermus thermophilus ribosome [42], the unmethylated G37 prevents the ASL from establishing the typical hydrogen-bond interaction between the O 2 atom of U32 and the N 6 atom of A38, thus rendering the nucleotides from 30 to 32 on the 5′-side of the ASL disordered and invisible (Fig. 4B).…”

“…4B). This mutant [ASL+G37.5] structure is prone to +1-frameshifting [42], most likely because the G37.5 nucleotide forms an aberrant base pair with U32 by displacing A38 away from the normal position (Fig. 4B).…”

Mechanisms of mRNA frame maintenance and its subversion during translation of the genetic code

“…Frameshift suppressor tRNAs have been invaluable tools to understand the general function of the ribosome despite the lack of clear molecular basis for their action. Recent structural studies shed light on the molecular basis of action of frameshift suppressor tRNAs that contain anticodon stem loop (ASL) insertions and also the roles of tRNA modifications in maintaining the reading frame (51,52). Although models to explain −1 and +1 frameshifting have been restricted to each class (50,(53)(54)(55)(56), single molecule and structural studies are beginning to provide molecular insights that indicate that each may not be as distinct as previously thought (29, 44-46, 51, 52).…”