ANALYSIS OF THE FINE-STRUCTURE OF THE PROHEAD BINDING DOMAIN OF THE PACKAGING PROTEIN OF BACTERIOPHAGE-T3 USING A HEXAPEPTIDE, AN ANALOG OFA PROHEAD BINDING-SITE

A large subunit of bacteriophage T3 packaging enzyme, a product of gen e 19 (gp19, 586 amino acid residues), binds a prohead prior to DNA tra nslocation in DNA packaging. Its C-terminal region (571 to 576, Region I) is of crucial importance for prohead binding. To elucidate the fun ctional role(s) of Region I in DNA packaging, a hexapeptide (6pT3) cor responding to the Region I sequence and its variants were synthesized and their effects on DNA packaging in a defined in vitro system were e xamined. 6pT3 did not inhibit gp19wt (wild type)-prohead binding but i nterfered with their functional interaction, resulting in inhibition o f DNA packaging. The inhibitory effect of 6pT3 on gp19wt was reversibl e. The effect of 6pT3 was examined with gp19 Delta C10, which was acti ve in DNA packaging in spite of lacking the extreme C-terminal 10 amin o acids (Region II). The inhibitory effect on gp19 Delta C10 was more severe than that on gpl9wt and was irreversible. From these results, w e concluded that the prohead binding domain is composed of two subdoma ins: Region I is a ''core'' domain, and its binding to the prohead is crucial for DNA packaging, and Region II is an ''anchor'' domain stabi lizing the binding by Region I. (C) 1995 Academic Press, Inc.