[Interaction between sodium bisulfite and bacteriophage SD DNA].

The interaction between sodium bisulfite and the cytosine residues within the intraphage DNA of phage SD was studied to elucidate the structure of viral nucleoprotein. Hydrolysis with perchloric acid of bisulfite-modified phage SD results in 18% decrease of cytosine and appearance of products having the properties of cytosyl amino acids (most probably cytosyl lysine). When the modified phage before hydrolysis was subjected to mild destruction in 0.1--1 M NaCl or Tris-HCl buffer (pH 7.0), neither the decrease of cytosine nor the appearance of cytosyl peptides was observed. However, these results were observed when the phage was heated at 70 degrees C in a medium containing 0.05 M phosphate buffer, pH 7.9--8.5. The presence of cytosyl amino acids in the modified phage, representing nucleotide-protein covalent cross-links explains the results of viscosometry and centrifugation in CS2SO4 density gradient. It is assumed that the bisulfite reaction with cytosine within phage SD is completed at the stage of intermediate product formation, i.e. C5--C6-dihydro-C6-sulfopyrimidine, in which the amino group is screened by interaction with protein (product VII). This product may exist only in situ; when the phage nucleoprotein is destroyed in phsophate-free media, product VII is converted into original cytosine. Under acidic hydrolysis or in the presence of phosphate ions under heating, product VII undergoes transamination accompanied by SO3 split-off and reconstitution of C5--C6 double bond to form cytosylamino acids.