Characterization of the DNA binding protein encoded by the N-specific filamentous Escherichia coli phage IKe. Binding properties of the protein and nucleotide sequence of the gene.

A DNA binding protein encoded by the filamentous single-stranded DNA phage IKe has been isolated from IKe-infected Escherichia coli cells. Fluorescence and in vitro binding studies have shown that the protein binds co-operatively and with a high specificity to single-stranded but not to double-stranded DNA. From titration of the protein to poly(dA) it has been calculated that approximately four bases of the DNA are covered by one monomer of protein. These binding characteristics closely resemble those of gene V protein encoded by the F-specific filamentous phages M13 and fd. The nucleotide sequence of the gene specifying the IKe DNA binding protein has been established. When compared to the nucleotide sequence of gene V of phage M13 it shows an homology of 58%, indicating that these two phages are evolutionarily related. The IKe DNA binding protein is 88 amino acids long which is one amino acid residue larger than the gene V protein sequence. When the IKe DNA binding protein sequence is compared with that of gene V protein it was found that 39 amino acid residues have identical positions in both proteins. The positions of all five tyrosine residues, a number of which are known to be involved in DNA binding, are conserved. Secondary structure predictions indicate that the two proteins contain similar structural domains. It is proposed that the tyrosine residues which are involved in DNA binding are the ones in or next to a beta-turn, at positions 26, 41 and 56 in gene V protein and at positions 27, 42 and 57 in the IKe DNA binding protein.