Mitochondrial DNA acts as potential promoter of the baculovirus RNA polymerase

We have examined whether mitochondrial DNA could act as target of the RNA polymerase encoded by the baculovirus Autographa californica multicapsid nuclear polyhedrosis virus, because the baculovirus late promoters and the control region of host mitochondrial DNA show a high degree of sequence similarity. In vitro transcription using mitochondrial DNA from Spodoptera frugiperda cells and nuclear extracts prepared from baculovirus infected cells demonstrates that mitochondrial DNA is recognized by the viral RNA polymerase. Transcriptional initiation occurs at TAAG sequences, although not all of the six TAAG motifs present in the mitochondrial DNA fragment are recognized. The TAAG motif in the control region served as weak transcriptional start site, but some of the TAAG motifs in the coding sequences of the adjacent tRNA and rRNA genes are recognized efficiently. The sequences flanking the TAAG motifs used as transcriptional start sites have a lower helix stability than the flanking sequences of the nonfunctional TAAG motifs. These results support the view that helix stability rather than sequence specificity is an important factor for recognition of TAAG motifs by the viral RNA polymerase.