Formation of Tertiary Interactions during rRNA GTPase Center Folding

Assembly factors chaperone ribosomal RNA folding by isolating helical junctions that are prone to misfolding

While RNAs are known to misfold, the underlying molecular causes have been mainly studied in fragments of biologically relevant larger RNAs. As these small RNAs are dominated by secondary structures, misfolding of these secondary structures remains the most-explored cause for global RNA misfolding. Conversely, how RNA chaperones function in a biological context to promote native folding beyond duplex annealing remains unknown. Here, in a combination of dimethylsulfate mutational profiling with sequencing (DMS-MaPseq), structural analyses, biochemical experiments, and yeast genetics, we show that three-helix junctions are prone to misfolding during assembly of the small ribosomal subunit in vivo. We identify ubiquitous roles for ribosome assembly factors in chaperoning their folding by preventing the formation of premature tertiary interactions, which otherwise kinetically trap misfolded junctions, thereby blocking further progress in the assembly cascade. While these protein chaperones act indirectly by binding the interaction partners of junctions, our analyses also suggest direct roles for small nucleolar RNAs (snoRNAs) in binding and chaperoning helical junctions during transcription. While these assembly factors do not utilize energy to ameliorate misfolding, our data demonstrate how their dissociation renders reversible folding steps irreversible, thereby driving native folding and assembly and setting up a timer that dictates the propensity of misfolded intermediates to escape quality control. Finally, the data demonstrate that RNA chaperones act locally on individual tertiary interactions, in contrast to protein chaperones, which globally unfold misfolded proteins.

Ribosomal Protein L11 Selectively Stabilizes a Tertiary Structure of the GTPase Center rRNA Domain

The GTPase Center (GAC) RNA domain in bacterial 23S rRNA is directly bound by ribosomal protein L11, and this complex is essential to ribosome function. Previous cocrystal structures of the 58-nucleotide GAC RNA bound to L11 revealed the intricate tertiary fold of the RNA domain, with one monovalent and several divalent ions located in specific sites within the structure. Here, we report a new crystal structure of the free GAC that is essentially identical to the L11-bound structure, which retains many common sites of divalent ion occupation. This new structure demonstrates that RNA alone folds into its tertiary structure with bound divalent ions. In solution, we find that this tertiary structure is not static, but rather is best described as an ensemble of states. While L11 protein cannot bind to the GAC until the RNA has adopted its tertiary structure, new experimental data show that L11 binds to Mg²⁺-dependent folded states, which we suggest lie along the folding pathway of the RNA. We propose that L11 stabilizes a specific GAC RNA tertiary state, corresponding to the crystal structure, and that this structure reflects the functionally critical conformation of the rRNA domain in the fully assembled ribosome.

Divalent ions tune the kinetics of a bacterial GTPase center rRNA folding transition from secondary to tertiary structure

Folding of an RNA from secondary to tertiary structure often depends on divalent ions for efficient electrostatic charge screening (nonspecific association) or binding (specific association). To measure how different divalent cations modify folding kinetics of the 60 nucleotide Ecoli rRNA GTPase center, we combined stopped-flow fluorescence in the presence of Mg²⁺, Ca²⁺, or Sr²⁺ together with time-resolved small angle X-ray scattering (SAXS) in the presence of Mg²⁺ to observe the folding process. Immediately upon addition of each divalent ion, the RNA undergoes a transition from an extended state with secondary structure to a more compact structure. Subsequently, specific divalent ions modulate populations of intermediates in conformational ensembles along the folding pathway with transition times longer than 10 msec. Rate constants for the five folding transitions act on timescales from submillisecond to tens of seconds. The sensitivity of RNA tertiary structure to divalent cation identity affects all but the fastest events in RNA folding, and allowed us to identify those states that prefer Mg²⁺ The GTPase center RNA appears to have optimized its folding trajectory to specifically utilize this most abundant intracellular divalent ion.

NMR Structures and Dynamics in a Prohead RNA Loop that Binds Metal Ions

Metal ions are critical for RNA structure and enzymatic activity. We present the structure of an asymmetric RNA loop that binds metal ions and has an essential function in a bacteriophage packaging motor. Prohead RNA is a noncoding RNA that is required for genome packaging activity in phi29-like bacteriophage. The loops in GA1 and phi29 bacteriophage share a conserved adenine that forms a base triple, although the structural context for the base triple differs. NMR relaxation studies and femtosecond time-resolved fluorescence spectroscopy reveal the dynamic behavior of the loop in the metal ion bound and unbound forms. The mechanism of metal ion binding appears to be an induced conformational change between two dynamic ensembles rather than a conformational capture mechanism. These results provide experimental benchmarks for computational models of RNA-metal ion interactions.

Nucleobases Undergo Dynamic Rearrangements during RNA Tertiary Folding

The tertiary structure of the GTPase center (GAC) of 23S ribosomal RNA (rRNA) as seen in cocrystals is extremely compact. It is stabilized by long-range hydrogen bonds and nucleobase stacking and by a triloop that forms within its three-way junction. Its folding pathway from secondary structure to tertiary structure has not been previously observed, but it was shown to require Mg²⁺ ions in equilibrium experiments. The fluorescent nucleotide 2-aminopurine was substituted at selected sites within the 60-nt GAC. Fluorescence intensity changes upon addition of MgCl₂ were monitored over a time-course from 1ms to 100s as the RNA folds. The folding pathway is revealed here to be hierarchical through several intermediates. Observation of the nucleobases during folding provides a new perspective on the process and the pathway, revealing the dynamics of nucleobase conformational exchange during the folding transitions.