ASSAYING PHAGE-BORNE PEPTIDES BY PHAGE CAPTURE ON FIBRINOGEN OR STREPTAVIDIN

There is no simple and efficient method for assaying phage isolated fr om libraries without having to resort to PEG purification of the phage , or to the biotinylation or other labelling of the target molecule, W e report here a method for producing 'bifunctional' phage that express two types of peptide; one peptide, fused to pVIII, will bind to immob ilized fibrinogen, allowing capture of the phage out of culture supern atants; this allows the other peptide, fused to pIII or pVIII to be as sayed by simple ELISA. This system has also been developed for the cap ture of phage bearing a streptavidin-binding peptide, The bifunctional phage are produced by bacterial cells bearing a plasmid that expresse s pVIII fused either to the fibrinogen-binding peptide or to the strep tavidin-binding one. Thus, when these cells are infected with a phage clone or pool to be assayed, phage will be produced whose 'capture-pep tide' is produced from the plasmid and whose 'assay-peptide' is produc ed from the phage genome. We show here that, by this method, bifunctio nal phage can be produced that will bind to immobilized streptavidin o r fibrinogen.