YihA has previously been characterized as an essential gene of unknown function in both Escherichia coli and Bacillus subtilis. It is conserved in bacteria and represents an attractive target for the discovery of new antibiotics. YihA encodes a putative GTP-binding protein. We have cloned and overexpressed the gene encoding E coli YihA and initiated biochemical studies as a first step towards understanding its biological function. We showed by circular dichroism that the purified protein has a secondary structure typical of most GTP-binding proteins. It binds guanine nucleotides specifically, as demonstrated by fluorescence resonance energy transfer between 2'-(or-3')-O-(N-methylanthraniloyl) nucleotides (mant-nucleotides) and the tryptophans of YihA. The K-d values for GDP and GTP were determined by competition with 2'-(or-3')-O-(N-methylanthraniloyl) GDP to be 3 and 27 muM, respectively. Using mutants of YihA we show that nucleotide binding occurs at the putative GTP-binding domain predicted from the primary sequence. (C) 2003 Elsevier Science (USA). All rights reserved.