Fig. 5: Model for H89 maturation.

H89 maturation requires the concerted action of GTPBP10 and GTPBP5. GTPBP10 has earlier been proposed to be part of a maturation intermediate in conjunction with the methyltransferase MRM3 and helicase DDX28. However, the low resolution of the reconstruction renders this interpretation ambiguous. We now find GTPBP10 in concert with GTPBP7 bound to the maturing ribosome when the pre-H68-71 rRNA stretch has already been deposited on the NSUN4-MTERF4 dimer (intermediate 1). Localization of H89 into its ribosomal cavity enables proper incorporation of uL16m and bL36m, which in turn induce GTP hydrolysis in GTPBP10 and its dissociation from the ribosome. GTPBP7 may then remain on the mtLSU that contains a partially unstructured H89 base (intermediate 2, Supplementary Fig. 10). uL16m incorporation may finally trigger GTP hydrolysis and reorganization of GTPBP7 on the RNA surface. This will vacate the necessary space for binding of the methyltransferase MRM2, which installs the methylation on U3039 in the A loop. As shown earlier, binding of GTPBP5 completes folding of the H89 base (intermediate 3), releases the A loop from MRM2, and may trigger departure of MRM2 and GTPBP7 from the complex.