Both rate zonal centrifugation and gel electrophoresis have revealed t
hat the mature double-stranded DNA of bacteriophage T5 has single-stra
nded breaks (nicks) at specific sites. Neither of these procedures has
previously revealed site-specific nicks in the double-stranded DNA of
other bacteriophages, including T7. In the present study, denaturing
gel electrophoresis, followed by specific DNA detection, reveals that
a small fraction of mature T7 DNA molecules, like most T5 DNA molecule
s, has site-specific nicks. The procedure of specific detection is to
probe with an oligonucleotide specific for one of the ends of T7 DNA.
If position 0.0 is the left genetic end and position 100.0 is the righ
t genetic end of T7 DNA, the nicks on the 5' left-oriented strand are
at 11.3, 12.4, 65.7, 79.2, and 86.0; the nicks on the 5' right-oriente
d strand are at 23.3 and 26.5 (+/- 0.5). The positions of the three ri
ghtmost nicks are indistinguishable from those of double-stranded brea
ks that produce previously demonstrated shorter than mature length DNA
s packaged in vivo. We propose that the T7 nicks are produced by prema
ture activity of the T7 terminase during DNA packaging. (C) 1995 Acade
mic Press, Inc.