Analysis of the termini of the DNA of bovine parvovirus: demonstration of sequence inversion at the left terminus and its implication for the replication model.

The distribution of terminal-sequence orientations in the viral DNA of bovine parvovirus (BPV), an autonomous parvovirus, was studied by end labeling and restriction enzyme digestion and also by cloning. The left (3') end of the minus strand of BPV was found in two alternative sequence orientations (designated as flip and flop, which are reverse complements of each other), with a 10-fold excess of flip. This is in contrast to the autonomous rodent parvoviruses which encapsidate minus-strand DNA with only the flip orientation at this end. The right (5') end of the minus strand of BPV contained both sequence orientations with equal frequencies, as in the rodent parvoviruses. Sequence inversions were also detected at both ends of the plus strand, which makes up about 10% of the encapsidated BPV DNA. Each terminus of BPV DNA had a characteristic ratio of flip to flop forms, and this ratio was restored in the progeny DNA resulting from transfection with genomic clones of different defined terminal conformations. Replicative-form DNA showed the same distribution of terminal-sequence orientations as the reannealed plus and minus virion DNAs, suggesting that the distribution of flip and flop forms observed in virion DNA is not due to selective encapsidation, but rather to the specific distribution of replicative forms. The current replication model for autonomous parvoviruses, which was based on the available data for the rodent parvoviruses, cannot account for the observed distribution of BPV DNA. An alternative model is suggested.     

Structure of a replication intermediate in the synthesis of Rous sarcoma virus DNA in vivo

Intermediates in the synthesis of Rous sarcoma virus DNA in vivo contain a short second strand of DNA (plus strong-stop DNA) synthesized by using the region near the 5' end of the first (minus) strand of DNA as the template. In this report, we show that the 3' end of plus strong-stop DNA is extended about 15 to 20 nucleotides beyond the 5' end of the minus-strand DNA template, probably copying a portion of the tRNATrp molecule that serves as primer for synthesis of the minus strand of DNA. The extra sequences present in plus strong-stop DNA may play a central role in the generation of the long terminal repeat present in mature forms of viral DNA.     

Heterogeneity of the 3'' end of minus-strand RNA in the poliovirus replicative form.

The 3' terminus of the strand (minus strand) complementary to poliovirion RNA (plus strand) has been examined to see whether this sequence extends to the 5'-nucleotide terminus of the plus strand, or whether minus-strand synthesis terminates prematurely, perhaps due to the presence of a nonreplicated nucleotide primer for initiation of plus-strand synthesis. The 3' terminus was labeled with 32P using [5'-32P]pCp and RNA ligase, and complete RNase digests were performed with RNases A, T1, and U2. 32P-oligonucleotides were analyzed for size by polyacrylamide-urea gel electrophoresis. The major oligonucleotide products formed were consistent with the minus strand containing 3' ends complementary and flush with the 5' end of the plus strand. However, a variable proportion of the isolated minus strands from different preparations were heterogeneous in length and appeared to differ from each other by the presence of one, two, or three 3'-terminal A residues.     

Protein covalently bound to minus-strand DNA intermediates of duck hepatitis B virus.

Analysis of duck hepatitis B viral DNA by gel electrophoresis, Southern blotting, and binding to benzoylated naphthoylated DEAE-cellulose showed that a protein is bound to the minus-strand virion DNA as well as to the full-length single strand, minus-strand species, and minus-strand DNA intermediates isolated from replicating complexes present in infected duck liver. By utilizing a modified dideoxynucleotidyl sequencing method, it was shown that the protein is covalently bound to the smallest detectable growing strands (ca. 30 bases) and that minus-strand synthesis begins at a unique site. These results support the notion that the protein may function as a primer for synthesis of the minus-strand DNA.     

Template switches during plus-strand DNA synthesis of duck hepatitis B virus are influenced by the base composition of the minus-strand terminal redundancy

Two template switches are necessary during plus-strand DNA synthesis of the relaxed circular (RC) form of the hepadnavirus genome. The 3' end of the minus-strand DNA makes important contributions to both of these template switches. It acts as the donor site for the first template switch, called primer translocation, and subsequently acts as the acceptor site for the second template switch, termed circularization. Circularization involves transfer of the nascent 3' end of the plus strand from the 5' end of the minus-strand DNA to the 3' end, where further elongation can lead to production of RC DNA. In duck hepatitis B virus (DHBV), a small terminal redundancy (5'r and 3'r) on the ends of the minus-strand DNA has been shown to be important, but not sufficient, for circularization. We investigated what contribution, if any, the base composition of the terminal redundancy made to the circularization process. Using a genetic approach, we found a strong positive correlation between the fraction of A and T residues within the terminal redundancy and the efficiency of the circularization process in those variants. Additionally, we found that the level of in situ priming increases, at the expense of primer translocation, as the fraction of A and T residues in the 3'r decreases. Thus, a terminal redundancy rich in A and T residues is important for both plus-strand template switches in DHBV.