cis-Acting sequences in addition to donor and acceptor sites are required for template switching during synthesis of plus-strand DNA for duck hepatitis B virus.

A characteristic of all hepadnaviruses is the relaxed-circular conformation of the DNA genome within an infectious virion. Synthesis of the relaxed-circular genome by reverse transcription requires three template switches. These template switches, as for the template switches or strand transfers of other reverse-transcribing genetic elements, require repeated sequences (the donor and acceptor sites) between which a complementary strand of nucleic acid is transferred. The mechanism for each of the template switches in hepadnaviruses is poorly understood. To determine whether sequences other than the donor and acceptor sites are involved in the template switches of duck hepatitis B virus (DHBV), a series of molecular clones which express viral genomes bearing deletion mutations were analyzed. We found that three regions of the DHBV genome, which are distinct from the donor and acceptor sites, are required for the synthesis of relaxed-circular DNA. One region, located near the 3' end of the minus-strand template, is required for the template switch that circularizes the genome. The other two regions, located in the middle of the genome and near DR2, appear to be required for plus-strand primer translocation. We speculate that these cis-acting sequences may play a role in the organization of the minus-strand DNA template within the capsid particle so that it supports efficient template switching during plus-strand DNA synthesis.

Activation of divergent neuronal cell death pathways in different target cell populations during neuroadapted sindbis virus infection of mice.

Infection of adult mice with neuroadapted Sindbis virus (NSV) results in a severe encephalomyelitis accompanied by prominent hindlimb paralysis. We find that the onset of paralysis parallels morphologic changes in motor neuron cell bodies in the lumbar spinal cord and in motor neuron axons in ventral nerve roots, many of which are eventually lost over time. However, unlike NSV-induced neuronal cell death found in the brain of infected animals, the loss of motor neurons does not appear to be apoptotic, as judged by morphologic and biochemical criteria. This may be explained in part by the lack of detectable caspase-3 expression in these cells.