Figure 1.

Ribosomal protein S1 is dispensable for the tRNA-like function of tmRNA. (A) Schematic representation of the first step of trans-translation (trans-peptidation) on polyU mRNAs. The light gray ovals are the ribosomal subunits (the larger for the 50S and the smaller for the 30S). The white ovals are, from right to left, the decoding site (A); the transpeptidyl-site (P) and the exit site (E). tRNAs and tmRNA are depicted as hooks and as a lasso, respectively (for further insights see (1)). (B) Western blots showing the quasi-absence of S1 in purified ‘S1-free’ 70S ribosomes compared to genuine 70S ribosomes. (C) The trans-translation in vitro assay is inactive in the absence of SmpB. (D) Assay of alanine trans-peptidation (asterisk in A), using S1-free 70S ribosomes and increasing amounts of exogenous S1. Trans-peptidation is monitored by the incorporation of ³H-Ala from alanyl-tmRNA into polyPhe and is normalized to the level of translation, measured in parallel by the incorporation of ¹⁴C-Phe (section ‘Experimental procedures’). The Y-axis represents the quantities (number of pmoles) of ³H alanine incorporated per assay.