This paper is available on line at http://www.jbc.org 33480 by guest on July 24, 2018 http://www.jbc.org/ Downloaded from clease-free Klenow were purchased from New England Biolabs; modified T7 DNA polymerase (Sequenase version 2.0) was from U. S. Biochemical Corp.; snake venom phosphodiesterase (SV) and P1 phosphodiesterase (P1), [␥-32 P]ATP, [␣-32 P]TTP, and [␣-32 P]dGTP (specific activity of 3,000 Ci/mmol) from Amersham Pharmacia Biotech; the C-18 HPLC column from Rainin; and nucleotides and deoxynucleotides from Life Technologies, Inc.