The Lrp/AsnC family of transcriptional regulatory proteins is found in both archaea and bacteria. Members of the family influence cellular metabolism in both a global (Lrp) and specific (AsnC) manner, often in response to exogenous amino acid effectors. In the present study we have determined both the first bacterial and the highest resolution structures for members of the family. Escherichia coli AsnC is a specific gene regulator whose activity is triggered by asparagine binding. Bacillus subtilis LrpC is a global regulator involved in chromosome condensation. Our AsnC-asparagine structure is the first for a regulator–effector complex and is revealed as an octameric disc. Key ligand recognition residues are identified together with a route for ligand access. The LrpC structure reveals a stable octamer supportive of a topological role in dynamic DNA packaging. The structures yield significant clues to the functionality of Lrp/AsnC-type regulators with respect to ligand binding and oligomerization states as well as to their role in specific and global DNA regulation.

The amino acid sequence and crystal structure of the ribosomal protein S6 from the small ribosomal subunit of Thermus thermophilus have been determined. S6 is a small protein with 101 amino acid residues. The 3D structure, which was determined to 2.0 A resolution, consists of a four‐stranded anti‐parallel beta‐sheet with two alpha‐helices packed on one side. Similar folding patterns have been observed for other ribosomal proteins and may suggest an original RNA‐interacting motif. Related topologies are also found in several other nucleic acid‐interacting proteins and based on the assumption that the structure of the ribosome was established early in the molecular evolution, the possibility that an ancestral RNA‐interacting motif in ribosomal proteins is the evolutionary origin for the nucleic acid‐interacting domain in large classes of ribonucleic acid binding proteins should be considered.

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Holliday junction resolution performed by a variety of structure-specific endonucleases is a key step in DNA recombination and repair. It is believed that all resolvases carry out their reaction chemistries in a similar fashion, utilizing a divalent cation to facilitate the hydrolysis of the phosphodiester backbone of the DNA, but their architecture varies. To date, with the exception of bacteriophage T4 endonuclease VII, each of the known resolvase enzyme structures has been categorized into one of two families: the integrases and the nucleases. We have now determined the structure of the Escherichia coli RusA Holliday junction resolvase, which reveals a fourth structural class for these enzymes. The structure suggests that dimer formation is essential for Mg(2+) cation binding and hence catalysis and that like the other resolvases, RusA distorts its Holliday junction target upon binding. Key residues identified by mutagenesis experiments are well positioned to interact with the DNA.

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