The Conserved GTPase LepA May Contribute to the Final Proper Stabilization of the 3′ Domain of the 30S Subunit During Ribosome Assembly

The function of the highly conserved GTPase LepA, a homolog of elongation factor EF-G, remains unknown in translation. However, there is biochemical data that it implicates in the 30S ribosomal subunit biogenesis. Here, using cryo-electron microscopy, we characterized 30S subunits isolated from an Escherichia coli strain with a deleted lepA gene. The cryo-EM maps for ∆lepA 30S particles were divided into classes corresponding to consecutive assembly intermediates: from particles characterized by unformed helices h44/h45 of the central decoding center (CDR) and highly flexible head, through intermediates with a distorted CDR and a partial stabilization of the head, to near-mature 30S subunits with correctly docked h44 in the CDR, accessible 3′ end of 16S rRNA for translation but significant flexibility in head domain. Cryo-EM analysis of ΔlepA 30S intermediates revealed that they predominantly proceed to nearly mature functional state and exhibit suboptimal flexibility in the head domain. This finding suggests that LepA likely contributes to the final proper stabilization of the 3′ domain of the 30S subunit during ribosome assembly.