Laser cross-linking of nucleic acids to proteins. Methodology and first applications to the phage T4 DNA replication system.

Single-pulse (approximately 8 ns) ultraviolet laser excitation of protein-nucleic acid complexes can result in efficient and rapid covalent cross-linking of proteins to nucleic acids. The reaction produces no nucleic acid-nucleic acid or protein-protein cross-links, and no nucleic acid degradation. The efficiency of cross-linking is dependent on the wavelength of the exciting radiation, on the nucleotide composition of the nucleic acid, and on the total photon flux. The yield of cross-links/laser pulse is largest between 245 and 280 nm; cross-links are obtained with far UV photons (200-240 nm) as well, but in this range appreciable protein degradation is also observed. The method has been calibrated using the phage T4-coded gene 32 (single-stranded DNA-binding) protein interaction with oligonucleotides, for which binding constants have been measured previously by standard physical chemical methods (Kowalczykowski, S. C., Lonberg, N., Newport, J. W., and von Hippel, P. H. (1981) J. Mol. Biol. 145, 75-104). Photoactivation occurs primarily through the nucleotide residues of DNA and RNA at excitation wavelengths greater than 245 nm, with reaction through thymidine being greatly favored. The nucleotide residues may be ranked in order of decreasing photoreactivity as: dT much greater than dC greater than rU greater than rC, dA, dG. Cross-linking appears to be a single-photon process and occurs through single nucleotide (dT) residues; pyrimidine dimer formation is not involved. Preliminary studies of the individual proteins of the five-protein T4 DNA replication complex show that gene 43 protein (polymerase), gene 32 protein, and gene 44 and 45 (polymerase accessory) proteins all make contact with DNA, and can be cross-linked to it, whereas gene 62 (polymerase accessory) protein cannot. A survey of other nucleic acid-binding proteins has shown that E. coli RNA polymerase, DNA polymerase I, and rho protein can all be cross-linked to various nucleic acids by the laser technique. The potential uses of this procedure in probing protein-nucleic acid interactions are discussed.