Photochemical cross-linking of the gene 5 protein.fd DNA complex from fd-infected cells

The gene 5 protein.fd DNA complex, which is an intermediate in the life cycle of fd bacteriophage, was isolated from fd-infected cells. After ultraviolet irradiation, approximately 10% of the gene 5 protein was covalently linked to fd DNA via the sulfhydryl group of the single cysteine residue at position 33 in the gene 5 protein. When intact, infected cells were first irradiated with ultraviolet light and the gene 5 protein.DNA complex subsequently isolated, less than 1% of the bound protein in the complex was found to be cross-linked. While these results agree with our previous findings based on irradiation of the gene 5 protein.fd DNA complex formed from purified gene 5 protein and fd DNA, they differ from the results of Lica and Ray ((1977) J. Mol. Biol. 115, 45-59) who found high yields of cross-linked gene 5 protein.fd DNA complex after ultraviolet irradiation of intact cells.