Effect of methylation of adenine N6 on kink turn structure depends on location.

N6-methyladenine is the most common covalent modification in cellular RNA species, with demonstrated functional consequences. At the molecular level this methylation could alter local RNA structure, and/or modulate the binding of specific proteins. We have previously shown that trans-Hoogsteen-sugar (sheared) A:G base pairs can be completely disrupted by methylation, and that this occurs in a sub-set ofD/D k-turn structures. In this work we have investigated to what extent sequence context affects the severity with which inclusion of N6-methyladenine into different A:G base pairs of a standard k-turn affects RNA folding and L7Ae protein binding. We find that local sequence has a major influence, ranging from complete absence of folding and protein binding to a relatively mild effect. We have determined the crystal structure of one of these species both free and protein-bound, showing the environment of the methyl group and the way the modification is accommodated into the k-turn structure.

The role of RNA structure in translational regulation by L7Ae protein in archaea.

A recent study has shown that archaeal L7Ae binds to a putative k-turn structure in the 5'-leader of the mRNA of its structural gene to regulate translation. To function as a regulator, the RNA should be unstructured in the absence of protein, but it should adopt a k-turn-containing stem-loop on binding L7Ae. Sequence analysis of UTR sequences indicates that their k-turn elements will be unable to fold in the absence of L7Ae, and we have demonstrated this experimentally in solution using FRET for the Archaeoglobus fulgidus sequence. We have solved the X-ray crystal structure of the complex of the A. fulgidus RNA bound to its cognate L7Ae protein. The RNA adopts a standard k-turn conformation that is specifically recognized by the L7Ae protein, so stabilizing the stem-loop. In-line probing of the natural-sequence UTR shows that the RNA is unstructured in the absence of L7Ae binding, but folds on binding the protein such that the ribosome binding site is occluded. Thus, L7Ae regulates its own translation by switching the conformation of the RNA to alter accessibility.

Sequence determinants of the folding properties of box C/D kink-turns in RNA.

Folding properties differ markedly between kink-turns (k-turns) that have different biological functions. While ribosomal and riboswitch k-turns generally fold into their kinked conformation on addition of metal ions, box C/D snoRNP k-turns remain completely unfolded under these conditions, although they fold on addition of L7Ae protein. Sequence elements have been systematically exchanged between a standard ribosomal k-turn (Kt-7) that folds on addition of metal ions, and a box C/D k-turn. Folding was studied using fluorescence resonance energy transfer and gel electrophoresis. Three sequence elements each contribute in an approximately additive manner to the different folding properties of Kt-7 and box C/D k-turns from archaea. Bioinformatic analysis indicates that k-turn sequences evolve sequences that suit their folding properties to their biological function. The majority of ribosomal and riboswitch k-turns have sequences allowing unassisted folding in response to the presence of metal ions. In contrast, box C/D k-turns have sequences that require the binding of proteins to drive folding into the kinked conformation, consistent with their role in the assembly of the box C/D snoRNP apparatus. The rules governing the influence of sequence on folding properties can be applied to other standard k-turns to predict their folding characteristics.

Control of box C/D snoRNP assembly by N6-methylation of adenine.

N6-methyladenine is the most widespread mRNA modification. A subset of human box C/D snoRNA species have target GAC sequences that lead to formation of N6-methyladenine at a key trans Hoogsteen-sugar A·G base pair, of which half are methylated in vivo The GAC target is conserved only in those that are methylated. Methylation prevents binding of the 15.5-kDa protein and the induced folding of the RNA Thus, the assembly of the box C/D snoRNP could in principle be regulated by RNA methylation at its critical first stage. Crystallography reveals that N6-methylation of adenine prevents the formation of trans Hoogsteen-sugar A·G base pairs, explaining why the box C/D RNA cannot adopt its kinked conformation. More generally, our data indicate that sheared A·G base pairs (but not Watson-Crick base pairs) are more susceptible to disruption by N6mA methylation and are therefore possible regulatory sites. The human signal recognition particle RNA and many related Alu retrotransposon RNA species are also methylated at N6 of an adenine that forms a sheared base pair with guanine and mediates a key tertiary interaction.