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J Virol, February 1998, p. 1186-1194, Vol. 72, No. 2
Department of Microbiology and
Immunology1 and
Mary Babb Randolph
Cancer Center,2 School of Medicine, West
Virginia University, Morgantown, West Virginia 26506
Received 20 May 1997/Accepted 5 November 1997
Two models for the mechanism of retroviral recombination have been
proposed: forced copy choice (minus-strand recombination) and strand
displacement-assimilation (plus-strand recombination). Each
minus-strand recombination event results in one template switch,
whereas each plus-strand recombination event results in two template
switches. Recombinant proviruses with one and more than one template
switches were previously observed. Recombinants with one template
switch were generated by minus-strand recombination, while recombinants
containing more than one template switch may have been generated by
plus-strand recombination or by correlated minus-strand recombination.
We recently observed that retroviral recombination exhibits high
negative interference whereby the frequency of recombinants containing
multiple template-switching events is higher than expected. To
delineate the mechanism that generates recombinants with more than one
template switch, we devised a system that permits only minus-strand
recombination. Two highly homologous vectors, WH204 and WH221,
containing eight different restriction site markers were used. The
primer binding site (PBS) of WH221 was deleted; although reverse
transcription cannot initiate from WH221 RNA, it can serve as a
template for DNA synthesis in heterozygotic virions. After one round of
retroviral replication, the structures of the recombinant proviruses
were examined. Recombinants containing two, three, four, and five
template switches were observed at 1.4-, 10-, 65-, and 50-fold-higher
frequencies, respectively, than expected. This indicates that
minus-strand recombination events are correlated and can generate
proviruses with multiple template switches efficiently. The frequencies
of recombinants containing multiple template switches were similar to
those observed in the previous system, which allowed both minus- and
plus-strand recombination. Thus, the previously reported high negative
interference during retroviral recombination can be caused by
correlated template switches during minus-strand DNA synthesis. In
addition, all examined recombinants contained an intact PBS, indicating
that most of the plus-strand DNA transfer occurs after completion of
the strong-stop DNA.
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Correlated Template-Switching Events during
Minus-Strand DNA Synthesis: a Mechanism for High Negative Interference
during Retroviral Recombination
*
Corresponding author. Mailing address: Mary Babb
Randolph Cancer Center, School of Medicine, West Virginia University,
Morgantown, WV 26506. Phone: (304) 293-5949. Fax: (304) 293-4667. E-mail: whu{at}wvumbrcc1.hsc.wvu.edu.
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