Lamotrigine-mediated rescue of RsgA-deficient reveals another role of IF2 in ribosome biogenesis.

Unlabelled: RsgA (small ribosomal subunit, 30S, GTPase), a late-stage biogenesis factor, releases RbfA from 30S-RbfA complex. Δ (deleted for ) shows a slow growth phenotype and an increased accumulation of 17S rRNA (precursor of 16S rRNA) and the ribosomal subunits. Here, we show that the rescue of the Δ strain by multicopy (IF2) is enhanced by simultaneous overexpression of initiator tRNA (i-tRNA), suggesting a role of initiation complex formation in growth rescue. The synergistic effect of IF2/i-tRNA is accompanied by increased processing of 17S rRNA (to 16S), and protection of the 16S rRNA 3'-minor domain. Importantly, we show that an IF2-binding anticonvulsant drug, lamotrigine (Ltg), also rescues the Δ strain growth. The rescue is accompanied by increased processing of 17S rRNA, protection of the 3'-minor domain of 16S rRNA, and increased 70S ribosomes in polysome profiles. However, Ltg becomes inhibitory to the strain whose growth was already rescued by an L83R mutation in . Interestingly, like wild-type , overproduction of Ltg alleles (having indel mutations in their domain II) also rescues the Δ strain (independent of Ltg). Our observations suggest the dual role of IF2 in rescuing the Δ strain. First, together with i-tRNA, IF2 facilitates the final steps of processing of 17S rRNA. Second, a conformer of IF2 functionally compensates for RsgA, albeit poorly, during 30S biogenesis.

Importance: RsgA is a late-stage ribosome biogenesis factor. Earlier, (IF2) was isolated as a multicopy suppressor of the Δ strain. How IF2 rescued the strain growth remained unclear. This study reveals that (i) the multicopy -mediated growth rescue of Δ and the processing of 17S precursor to 16S rRNA in the strain are enhanced upon simultaneous overexpression of initiator tRNA and (ii) a conformer of IF2, whose occurrence increases when IF2 is overproduced or when Δ is treated with Ltg (an anticonvulsant drug that binds to domain II of IF2), compensates for the function of RsgA. Thus, this study reveals yet another role of IF2 in ribosome biogenesis.