YsxC is a placeholder for ribosomal protein uL2 during 50S ribosomal subunit assembly

The maturation of the functional core of the 50S ribosomal subunit in Bacillus subtilis is assisted by assembly factors that enhance the efficiency of the process. Two essential assembly factors, the GTPases RbgA and YphC, bind at or near the functional sites of the 50S subunit to promote the folding of ribosomal RNA helices that play key functional roles. YsxC is another GTPase involved in the maturation of the 50S subunit, whose function remains unknown. We demonstrate that YsxC aids 50S assembly through a drastically different mechanism. YsxC binds in the body of the 44.5S large ribosome assembly intermediate, occupying the site where uL2 binds and controls the timing in the folding of rRNA helices forming the binding site for uL2. It creates a "primordial" binding site that includes six out of the eleven rRNA helices forming the uL2 mature binding site. Once YsxC is released, uL2 binds to this "primordial" binding site, and the remaining helices that stabilize uL2 fold, and the entire region adopts the mature conformation. This role of YsxC functioning as a placeholder factor for ribosomal protein uL2 provides the first example of such a factor's involvement in the ribosome assembly process in bacteria.