Gene V protein of fd bacteriophage. Dimer formation and the role of tyrosyl groups in DNA binding.

Gene V protein exists principally as a dimer in neutral buffers which are 0.15 M in NaCl, NaF, or NaClO4. Higher concentrations of NaClO4 or NaCl disrupt the dimers, but higher concentrations of NaF do not. There is a single sulfhydryl group per gene V protein monomer in native monomers, dimers, and DNA-protein complexes. Disulfide formation leads to loss of protein solubility and DNA binding capacity. The fluorescence of tyrosyl groups is the same for monomers and dimers in NaCl and NaClO4 solutions, but it is extensively quenched on binding to poly(dT) and fd-DNA. Complexes of gene V protein and fd-DNA isolated from lysates of infected cells were found to contain 4.70- +/- 0.13 nucleotides per monomer of gene V protein whereas complexes formed in vitro contain 4.05 +/- 0.17 nucleotides/monomer. It is postulated that tyrosyl groups are not involved in the protein-protein interactions of the monomer-dimer equilibrium, that tyrosyl groups stack with DNA bases in the complexes, and that each subunit of gene V protein in the intracellular complexes with fd-DNA is replaced by exactly two subunits of major coat protein during final assembly of the virus.