Fig. 7: Structure-based mechanistic model for SufB2-induced +1 frameshifting.

A SufB2-TC uses triplet anticodon-codon pairing in the 0-frame at a CCC-C motif, undergoes peptide-bond formation, and enables the resulting PRE complex to undergo a GS1 → GS2 transition, all with rates similar to those of ProL-TC. During the GS1 → GS2 transition, the 30S subunit rotates relative to the 50S subunit by 8° in the counter-clockwise (+) direction along the black curved arrow³⁸; the 30S subunit head swivels relative to the 30S subunit body by 5° in the clockwise (–) direction against the black curved arrow; the L1 stalk closes by ~60 Å; and the tRNAs are reconfigured from their P/P and A/A to their P/E and A/P configurations. EF-G then binds to the PRE complex to form PRE-G1 and subsequently catalyzes a series of conformational rearrangements of the complex (PRE-G1 to PRE-G4) that encompass further counter-clockwise and clockwise rotations of the subunits; severing of decoding center interactions with the anticodon-codon duplex in the A site; counter-clockwise and clockwise swiveling of the head and the associated opening and closing of the E-site gate; opening of the L1 stalk; and reconfigurations of the tRNAs as they move from the P and A sites to the E and P sites. It is during these steps, shown in red arrows within the gray shaded box, that SufB2 impedes forward and/or reverse swiveling of the head and the associated opening and/or closing of the E-site gate, facilitating +1 frameshifting. Next, EF-G and the deacylated tRNA dissociate from PRE-G4, leaving a POST complex ready to enter the next elongation cycle. The cartoons depicting PRE-G1(GS1) and PRE-G1(GS2) were generated using Biological Assemblies 2 and 1, respectively, of PDB entry 4V9D. Due to the lack of an A-site tRNA or EF-G in 4V9D, cartoons of the A- and P-site tRNAs from the previous structures¹ were positioned into the two assemblies using the P-site tRNAs in 4V9D as guides and a cartoon of EF-G generated from 4V7D was manually positioned near the factor binding site of the ribosomes. The cartoons depicting PRE-G2, PRE-G3, and PRE-G4 were generated from 4V7D, 4W29, and 4V5F, respectively, and colored as in Fig. 6, with the head domain shown in orange.