NfrA of Escherichia coli was found to be one of the two genes whose products are required for bacteriophage N4 adsorption. The nfrA gene encodes a 990-residue long outer membrane protein which presumably serves as the phage receptor, whereas the nfrB gene encodes an inner membrane protein and may be a component of the receptor. Besides acting as receptor for N4 bacteriophage, the physiological role of nrfA is unknown. The transmembrane domain of the protein is located in the C-terminus whereas a large N-terminal periplasmic domain of unknown function (TPR) domain, which is known to be associated with protein-protein interactions, is also present.