Protein and Mg2+-induced conformational changes in the S15 binding site of 16 S ribosomal RNA

The Bacillus stearothermophilus ribosomal protein S15 binds to the central domain of the 16 S rRNA inducing a conformational change in a three-way helical junction. To understand the nature of this conformational change, extended-helical junctions were prepared to examine the effects of S15 or Mg2+ binding on the relative helical orientation using native gel electrophoretic mobility and transient electric birefringence. The free junction is planar with similar to 120 degrees interhelical angles, whereas S15 and Mg2+ yield a junction conformation that remains planar in which two helices, 21 and 22, become colinear and the third, helix 20, forms a 60 degrees angle with respect to helix 22. This conformational change is thought to be important for directing the assembly of the central domain of the 30 S ribosomal subunit. (C) 1998 Academic Press Limited.