Functional role of the C-terminal tail of the archaeal ribosomal stalk in recruitment of two elongation factors to the sarcin/ricin loop of 23S rRNA.

Two types of elongation factors alternate in their binding to the factor-binding center of the ribosome. Both binding events are accompanied by GTP hydrolysis and drive the translation elongation cycle. The multicopy ribosomal protein family, termed the stalk, contributes actively to the elongation process. Recent evidence indicates that the mobile C-terminal tail of archaeal stalk aP1 directly interacts with both the elongation factors aEF1A and aEF2. To investigate the functional significance of these interactions in recruitment of elongation factors to the factor-binding center of the ribosome, we substituted the archaeal stalk complex aL10•aP1 for the bL10•bL12 stalk complex in the Escherichia coli 50S subunit. The resultant hybrid ribosome accessed archaeal aEF1A and aEF2 in a manner dependent on the C-terminal tail containing the hydrophobic residues Leu103, Leu106 and Phe107. Bases G2659 and A2660 in the sarcin/ricin loop (SRL) of 23S rRNA were protected against DMS modification by both factors as was A1067 by aEF2. Mutagenesis indicated that this protection was dependent on the intact C-terminal tail of aP1. The results suggest a crucial role for the interactions between the stalk C-terminal tail and elongation factors in their recruitment to the SRL of 23S rRNA within the ribosome.