The protelomerase of temperate Escherichia coli phage N15 has cleaving-joining activity

Escherichia coli phage N15 encodes the slightly acidic, 630-residue protein of 72.2 kDa called protelomerase (TelN). TelN is a component of the N15 re plication system proposed to be involved in the generation of the linear pr ophage DNA. This linear DNA molecule has covalently closed ends. The reacti on converting circular plasmids into linear molecules was catalyzed in vitr o. We demonstrate that the product of telN functions as the protelomerase i n the absence of other N15-encoded factors. Purified TelN processes circula r and linear plasmid DNA containing the proposed target site telRL to produ ce linear double-stranded DNA with covalently closed ends. The 56-bp telRL target site consists of a central telO palindrome of 22 bp and two 14-bp fl anking sequences comprising inverted repeats. telO is separated from these repeats by 3 bp on each side. The telRL sequence is sufficient for TelN-med iated processing. The ends of the DNA molecules generated in vitro have the same configuration as do those observed in vivo. TelN exerts its activity as cleaving-joining enzyme in a concerted action.