7 Strand Displacement Synthesis by Reverse Transcriptase

Abstract

We discuss here a property of reverse transcriptase revealed by observations made some time ago that unintegrated linear avian sarcoma-leukosis virus (ASLV) DNA molecules have a unique structure. The unintegrated linear ASLV DNA consists of a continuous minus strand that is complementary to the RNA genome and a discontinuous (i.e., segmented) plus strand from which single-stranded branches have been created by strand-displacement synthesis of adjacent downstream plus-strand segments (Fig. 1). In most generic models for retroviral DNA synthesis, strand displacement is proposed as the means by which the plus-strand strong-stop DNA is displaced from its template and copied a second time, generating a long terminal repeat (LTR) duplication (Fig. 2) (see, e.g., Gilboa et al. 1979a). However, a discontinuous plus strand has not been featured in most of these models, and thus the possible consequences of strand-displacement synthesis involving plus-strand segments have not usually been considered. There may be important biological consequences due to the presence of single-stranded branches of plus-strand DNA, and electron microscopy observations have led to the proposal of a displacement/assimilation model for retroviral recombination. In this chapter, we review the available data concerning the nature of plus strands synthesized by a variety of retroviruses in permeabilized virions and in infected cells. We also examine the evidence for strand-displacement synthesis by the reverse transcriptase of ASLV and discuss the possible implications of these findings for retroviral recombination.

DISCONTINUOUS PLUS STRANDS IN UNINTEGRATED LINEAR RETROVIRAL DNA
The presence of a discontinuous plus strand in unintegrated ASLV DNA is...