J Biol Chem, Vol. 273, Issue 49, 32568-32575, December 4, 1998
Analysis of Polypurine Tract-associated DNA Plus-strand
Priming in Vivo Utilizing a Plant Pararetroviral Vector
Carrying Redundant Ectopic Priming Elements
Rob J.
Noad,
Nadia S.
Al-Kaff,
David S.
Turner, and
Simon N.
Covey
From the John Innes Centre, Norwich Research Park, Colney,
Norwich NR4 7UH, United Kingdom
Initiation of DNA plus-strand synthesis in most
reverse-transcribing elements requires primer generation by reverse
transcriptase-associated RNase H at one or more template polypurine
tracts (PPTs). We have exploited infectious clones of the plant
pararetrovirus cauliflower mosaic virus carrying redundant ectopic
plus-strand priming elements to study priming in vivo.
Ectopic priming generated an additional discontinuity in progeny virion
DNA during infection of plants. We found that altering the length of
the 13-base pair PPT by ±25% significantly reduced priming
efficiency. A short pyrimidine tract 5' to the PPT, highly conserved
among diverse reverse-transcribing elements, was shown to play an
important role in PPT recognition in vivo. The predominant
DNA plus-strand 5' end remained 3 nucleotides from the PPT 3' end in
mutant primers that were longer or shorter than the wild-type primer.
Use of an ectopic redundant primer to study
replication-dependent priming was validated by
demonstrating that it could rescue infectivity following destruction of
the wild-type priming elements. We propose a model for plant
pararetroviral plus-strand priming in which pyrimidines enhance
PPT recognition during polymerase-dependent RNase H
cleavages, and suggest that fidelity of primer maturation during
polymerase-independent cleavages involves PPT length measurement and 3'
end recognition by RNase H.
Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.