The yrdC family of genes codes for proteins that occur both independently and as a domain in proteins that have been implicated in regulation. An example for the latter case is the sua5 gene from yeast. Sua5 was identified as a suppressor of a translation initiation defect in cytochrome c and is required for normal growth in yeast (Na JG, Pinto I, Hampsey M, 1992, Genetics 11:791–801). However, the function of the Sua5 protein remains unknown; Sua5 could act either at the transcriptional or the posttranscriptional levels to compensate for an aberrant translation start codon in the cyc gene. To potentially learn more about the function of YrdC and proteins featuring this domain, the crystal structure of the YrdC protein from Escherichia coli was determined at a resolution of 2.0 Å. YrdC adopts a new fold with no obvious similarity to those of other proteins with known three-dimensional (3D) structure. The protein features a large concave surface on one side that exhibits a positive electrostatic potential. The dimensions of this depression, its curvature, and the fact that conserved basic amino acids are located at its floor suggest that YrdC may be a nucleic acid binding protein. An investigation of YrdC's binding affinities for single- and double-stranded RNA and DNA fragments as well as tRNAs demonstrates that YrdC binds preferentially to double-stranded RNA. Our work provides evidence that 3D structures of functionally uncharacterized gene products with unique sequences can yield novel folds and functional insights.