Escherichia coli ykfE ORFan gene encodes a potent inhibitor of C-type lysozyme

The complete nucleotide sequences of over 37 microbial and three eukaryote genomes are already publicly available, and more sequencing is in progress. Despite this accumulation of data, newly sequenced microbial genomes conti nue to reveal up to 50% of functionally uncharacterized "anonymous" genes. A majority of these anonymous proteins have homologues in other organisms, whereas the rest exhibit no clear similarity to any other sequence in the d ata bases. This set of unique, apparently species-specific, sequences are r eferred to as ORFans, The biochemical and structural analysis of ORFan gene products is of both evolutionary and functional interest. Here we report t he cloning and expression of Escherichia coli ORFan ykfE gene and the funct ional characterization of the encoded protein. Under physiological conditio ns, the protein is a homodimer with a strong affinity for C-type lysozyme, as revealed by co-purification and co-crystallization. Activity measurement s and fluorescence studies demonstrated that the YkfE gene product is a pot ent C-type lysozyme inhibitor (K-i approximate to 1 nM), To denote this new ly assigned function, ykfE has now been registered under the new gene name Ivy (inhibitor of vertebrate lysozyme) at the E, coli genetic stock center.