Fig. 7: Overall mechanism of THB inhibition of bacterial translation cycle.

THB binds tightly to the 70S IC upon subunit joining but has no effect on initiator tRNA binding to the 30S pre-IC and subsequent 70S IC formation. THB impedes EF-Tu•GTP mediated delivery of the aa-tRNA to the A site and its subsequent accommodation into PTC during translation elongation. However, THB permits ribosomal subunit rotation, EF-G binding and translocation of peptidyl-tRNA from the A- to the P-site. During termination, THB inhibits the binding of RF1/RF2 to the stop codon at the A site of pre-TC and subsequent conformational change in the RFs required for peptide release. Further, THB prevents the splitting of post-TC into subunits by RRF and EF-G. This effect primarily originates from the ‘gluing’ effect of THB, which tethers the subunits together. This effect also causes re-association of 70S from already dissociated subunits in the absence of IF3. The red signs indicate the steps THB inhibits during translation cycle, while the purple signs indicate the steps impeded by THB. All other steps including initiation and translocation remain unaffected by THB.