SEQUENCE IDENTITY OF THE TERMINAL REDUNDANCIES ON THE MINUS-STRAND DNA-TEMPLATE IS NECESSARY BUT NOT SUFFICIENT FOR THE TEMPLATE SWITCH DURING HEPADNAVIRUS PLUS-STRAND DNA-SYNTHESIS

The template for hepadnavirus plus-strand DNA synthesis is a terminall y redundant minus-strand DNA. An intramolecular template switch during plus-strand DNA synthesis, which permits plus strand DNA elongation, has been proposed to be facilitated by this terminal redundancy which is 7 to 9 nucleotides long. The aim of this study was to determine whe ther the presence of identical copies of the redundancy on the minus-s trand DNA template was necessary and/or sufficient for the template sw itch and at what position(s) within the redundancy the switch occurs f or duck hepatitis B virus. When dinucleotide insertions were placed wi thin the copy of the redundancy at the 3' end of the minus-strand DNA template, novel sequences were copied into plus-strand DNA. The genera tion of these novel sequences could be explained by complete copying o f the redundancy at the 5' end of the minus-strand DNA template follow ed by a template switch and then extension from a mismatched 3' termin us. In a second set of experiments, it was found that when one copy of the redundancy had either three or five nucleotides replaced the temp late switch was inhibited. When the identical, albeit mutant, sequence s were restored in both copies of the redundancy, template switching w as not necessarily restored. Our results indicate that the terminal re dundancy on the minus-strand DNA template is necessary but not suffici ent for template snitching.