Ribosomal protein S21 is required for full activity in the initiation of protein synthesis.

Abstract

The role of the 30 S ribosomal protein S 21 in ribosome function, specifically in relation to the initiation of polypeptide synthesis, was studied. Reconstituted particles using 16 S RNA and all purified 30 S proteins, but omitting S21. The reconstituted particles ([-S 21] particles) sedimented at 30 S and contained all the 30 S proteins except S 21 in approximately the same concentration as control reconstituted particles ([Σ Si] particles). In several functional assays including AUG-directed fMet-tRNA binding, [-S 21] particles were only 33 to 44% as active as complete particles. The addition of pure S 21 to [-S 21] particles showed 2 to 3 fold stimulation in AUG-directed fMet-tRNA binding using purified initiation factors IF 1 and IF2, assayed either in the presence or absence of 50 S subunits. Maximum stimulation was observed when approximately 0.5 to 1.0 mole of S 21 per mole of [-S 21] particles were added. We conclude that, contrary to the report of van Duin et al. (1972), S 21 does not inhibit AUG-directed fMet-tRNA binding, and that S 21 is required for full activity of the ribosomes in the initiation of polypeptide synthesis. Thus, the results published by van Duin et al. cannot be taken to support the postulated heterogeneity of the ribosome population in vivo. Possible reasons for the discrepancy between our conclusion and that of van Duin et al. are discussed.