Analysis of the fine structure of the prohead binding domain of the packaging protein of bacteriophage T3 using a hexapeptide, an analog of a prohead binding site.

A large subunit of bacteriophage T3 packaging enzyme, a product of gene 19 (gp19, 586 amino acid residues), binds a prohead prior to DNA translocation in DNA packaging. Its C-terminal region (571 to 576, Region I) is of crucial importance for prohead binding. To elucidate the functional role(s) of Region I in DNA packaging, a hexapeptide (6pT3) corresponding to the Region I sequence and its variants were synthesized and their effects on DNA packaging in a defined in vitro system were examined. 6pT3 did not inhibit gp19wt (wild type)-prohead binding but interfered with their functional interaction, resulting in inhibition of DNA packaging. The inhibitory effect of 6pT3 on gp19wt was reversible. The effect of 6pT3 was examined with gp19 delta C10, which was active in DNA packaging in spite of lacking the extreme C-terminal 10 amino acids (Region II). The inhibitory effect on gp19 delta C10 was more severe than that on gp19wt and was irreversible. From these results, we concluded that the prohead binding domain is composed of two subdomains: Region I is a "core" domain, and its binding to the prohead is crucial for DNA packaging, and Region II is an "anchor" domain stabilizing the binding by Region I.