Definition of bases in 23S rRNA essential for ribosomal subunit association

The ribosome is a two-subunit molecular machine, sporting a working cycle that involves coordinated movements of the subunits. Recent structural studies of the 70S ribosome describe a rather large number of intersubunit contacts, some of which are dynamic during translocation. We set out to determine which intersubunit contacts are functionally indispensable for the association of ribosome subunits by using a modification interference approach. Modification of the N-1 position of A715, A1912, or A1918 in Escherichia coli SOS subunits is strongly detrimental to 70S ribosome formation. This result points to 23S rRNA helices 34 and 69, and thus bridges B2a and B4, as essential for ensuring stability of the 70S ribosome.