Studies of coronavirus DI RNA replication using in vitro constructed DI cDNA clones.

Sequence analysis of an intracellular defective-interfering (DI) RNA, DIssE, of mouse hepatitis virus (MHV) revealed that it is composed of three noncontiguous genomic regions, representing the first 864 nucleotides of the 5'-end, an internal 748 nucleotides of the polymerase gene, and 601 nucleotides from the 3'-end of the parental MHV genome. DIssE had three base substitutions within the leader sequence and also a deletion of nine nucleotides located at the junction of the leader and the remaining genomic sequence. A system was developed for generating DI RNAs to study the mechanism of MHV RNA replication. A cDNA copy of DIssE RNA was placed downstream of T7 RNA polymerase promoter to generate DI RNAs capable of extremely efficient replication in the presence of a helper virus. We demonstrated that, in the DI RNA-transfected cells, the leader sequence of these DI RNAs was switched to that of the helper virus during one round of replication. This high-frequency leader sequence exchange was not observed if a nine-nucleotide stretch at the junction between the leader and the remaining DI sequence was deleted. This observation suggests that a free leader RNA is utilized for the replication of MHV RNA.