Infection of the unicellular, eukaryotic Chlorella-like green alga NC64A by the large dsDNA containing virus PBCV-1 immediately reduced host RNA synthesis. Chloroplast rRNAs, but not cytosolic rRNAs, were degraded following viral infection. Northern blot analysis utilizing four cloned fragments of PBCV-1 DNA as probes, which represent about 12% of the viral genome, revealed several properties of PBCV-1 transcription: (i) A few viral transcripts were detected within 5 min after infection. (ii) Each PBCV-1 DNA clone hybridized to both early and late transcripts which implies that early and late genes are dispersed throughout the viral genome. (iii) The transition from early to late transcription occurred between 40 and 60 min after infection coincident with the onset of viral DNA synthesis. (iv) Three of the four DNA clones hybridized to transcripts which additively were larger than the corresponding DNA probe. This could reflect RNA processing, presence of overlapping genes, or transcription from both DNA strands. (v) A few, but not all, early transcripts were synthesized in the presence of cycloheximide. This suggests that the virus either carries in its own RNA polymerase or uses a host RNA polymerase for very early viral transcription and that synthesis of additional, later transcripts depends on translation of an early gene product(s).
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