SUMMARY Sequence-dependent recognition of dsDNA-binding proteins are well understood, yet sequence-specific recognition of dsRNA by proteins remains largely unknown, despite their importance in RNA maturation pathways. Adenosine deaminases that act on RNA (ADARs) recode genomic information by the site-selective deamination of adenosine. Here, we report the solution structure of the ADAR2 double-stranded RNA-binding motifs (dsRBMs) bound to a stem-loop pre-mRNA encoding the R/G editing site of GluR-2. The structure provides a molecular basis for how dsRBMs recognize the shape, and also more surprisingly, the sequence of the dsRNA. The unexpected direct readout of the RNA primary sequence by dsRBMs is achieved via the minor groove of the dsRNA and this recognition is critical for both editing and binding affinity at the R/G site of GluR-2. More generally, our findings suggest a solution to the sequence-specific paradox faced by many dsRBM-containing proteins that are involved in post-transcriptional regulation of gene expression.

Recruitment of appropriate RNA processing factors to the site of transcription is controlled by post-translational modifications of the C-terminal domain (CTD) of RNA polymerase II (RNAP II). Here, we report the solution structure of the Ser5 phosphorylated (pSer5) CTD bound to Nrd1. The structure reveals a direct recognition of pSer5 by Nrd1 that requires the cis conformation of the upstream pSer5-Pro6 peptidyl-prolyl bond of the CTD. Mutations at the complex interface diminish binding affinity and impair processing or degradation of noncoding RNAs. These findings underpin the interplay between covalent and noncovalent changes in the CTD structure that constitute the CTD code.

We analyzed DNA duplexes modified at central guanine residues by monofunctional Ru(II) arene complexes [(eta(6)-arene)Ru(II)(en)(Cl)](+) (arene = tetrahydroanthracene or p-cymene, Ru-THA or Ru-CYM, respectively). These two complexes were chosen as representatives of two different classes of Ru(II) arene compounds for which initial studies revealed different binding modes: one that may involve DNA intercalation (tricyclic-ring Ru-THA) and the other (mono-ring Ru-CYM) that may not. Ru-THA is approximately 20 times more toxic to cancer cells than Ru-CYM. The adducts of Ru-THA and Ru-CYM have contrasting effects on the conformation, thermodynamic stability, and polymerization of DNA in vitro. In addition, the adducts of Ru-CYM are removed from DNA more efficiently than those of Ru-THA. Interestingly, the mammalian nucleotide excision repair system has low efficiency for excision of ruthenium adducts compared to cisplatin intrastrand crosslinks.

PA-IIL is a fucose-binding lectin from Pseudomonas aeruginosa that is closely related to the virulence factors of the bacterium. Previous structural studies have revealed a new carbohydrate-binding mode with direct involvement of two calcium ions (Mitchell E, Houles C, Sudakevitz D, Wimmerova M, Gautier C, Perez S, Wu AM, Gilboa-Garber N, Imberty A. Structural basis for selective recognition of oligosaccharides from cystic fibrosis patients by the lectin PA-IIL of Pseudomonas aeruginosa. Nat Struct Biol 2002;9:918-921). A combination of thermodynamic, structural, and computational methods has been used to study the basis of the high affinity for the monosaccharide ligand. A titration microcalorimetry study indicated that the high affinity is enthalpy driven. The crystal structure of the tetrameric PA-IIL in complex with fucose and calcium was refined to 1.0 A resolution and, in combination with modeling, allowed a proposal to be made for the hydrogen-bond network in the binding site. Calculations of partial charges using ab initio computational chemistry methods indicated that extensive delocalization of charges between the calcium ions, the side chains of the protein-binding site and the carbohydrate ligand is responsible for the high enthalpy of binding and therefore for the unusually high affinity observed for this unique mode of carbohydrate recognition.