Abstract— The DNA of bacteriophage SP02c12 was subjected to 254 nm irradiation in solutions containing lysozyme or histone. In these solutions, the protein‐DNA mass ratios and the ionic strengths of the solvents were varied to change the amount of protein associated with the DNA. Lysozyme‐DNA binding constants were measured under the same conditions. The sensitivity of phage DNA to biological inactivation by UV increased as the amount of lysozyme bound per DNA strand increased. Although binding constants could not be measured for the DNA‐histone interaction, this protein had a protective effect which was greater under conditions which cause enhanced binding. No crosslinking of either protein could be detected even at doses ten‐fold greater than those giving a surviving fraction of 0.01.
Irradiation was also performed in the presence of various amino acids and short peptides. These were chosen to include amino acids which: (1) are positively charged, (2) absorb UV of this wavelength or (3) form UV‐induced crosslinks to DNA. None of the amino acids tested affected sensitivity of the DNA to biological inactivation. Peptides containing a UV‐absorbing amino acid and a positively charged amino acid enhanced sensitivity. For each of these peptides, a mixture of the constituent amino acids had the same effect as the peptide itself. Under the conditions used, no evidence for formation of DNA‐amino acid crosslinks was found. The results indicate that proteins and peptides can sensitize DNA to UV inactivation by mechanisms other than covalent crosslink formation. Such mechanisms could include energy or electron transfer or alterations in the conformation of the DNA.
